The  Earth 

Its  Life  and  Death 


By 

Alphonse  Berget 

Professor  at  the  Institut  Oceanographique 


Translated  by 
E.  W.  Barlow,  B.Sc.,  F.R.A.S. 


G.  P.  Putnam's  Sons 

New  York  and  London 
fmfcfcetbocfcer    pces0 
1915 


COPYRIGHT,  1915 

BY 

G.  P.  PUTNAM'S  SONS 


Ube  •fcnfcfeerbocfcer  ftosg,  Hew  ffiorfe 


PREFACE 

EVERYTHING  on  the  surface  of  our  Earth 
appears  to  us  in  living  form.  Terrestrial 
or  aquatic  animals,  insects,  plants,  microscopic 
organisms — everything  is  born,  lives,  and  dies. 
Matter  itself  is  subject  to  evolution  and,  under 
certain  conditions,  exhibits  distinct  signs  of  age 
and  fatigue. 

The  question  arises:  Does  the  Earth,  taken  as 
a  whole,  follow  this  general  law?  Does  it  live  in 
a  way  analogous  to  that  in  which  all  things  found 
on  its  surface  do?  These  pages  have  been  written 
to  answer  this  question.  I  trust  that  they  will 
give  to  those  who  read  them  a  general  idea  of  that 
science  which  may  be  called  the  physics  of  the 
Earth,  a  science  which  deserves,  in  the  highest 
meaning  of  the  word,  the  beautiful  name  of  <l>uai<;. 

A.  B. 

LE   POULIGUER. 


iii 


333911 


NOTE 

FOR  the  convenience  of  English  readers,  the 
approximate  equivalents  of  the  metric  figures 
are  in  this  volume  given  also  in  the  more  familiar 
English  standards. 


CONTENTS 

CHAPTER  PAGE 

I. — THE  BIRTH  OF  THE  EARTH          .         .         i 
II. — THE  AGE  OF  THE  EARTH    ...      24 

III. — THE  FORM,  MAGNITUDE,  AND  MASS  OF 

THE  EARTH          ....       56 

IV. — THE  MOVEMENTS  OF  THE  EARTH         .       90 
V. — THE  FORCE  OF  GRAVITY     .         .         .127 

VI. — THE  RHYTHMIC  MOVEMENTS  OF  THE 
EARTH'S  CRUST.  DEVIATIONS  OF 
THE  VERTICAL  .  .  .  .  173 

VII. — THE  SUDDEN  MOVEMENTS  OF  THE 
EARTH'S  CRUST.  SEISMIC  PHE- 
NOMENA .  .  Y  .  .  192 

VIII. — THE    MAGNETISM,    ELECTRICITY,    AND 

RADIOACTIVITY  OF  THE  EARTH      .     230 

IX. — THE   RHYTHMIC   MOVEMENTS   OF    THE 

OCEAN,  TIDES,  SWELL,  AND  WAVES    276 

X. — THE    CIRCULATION    OF    THE    EARTH, 

MARINE  AND  ATMOSPHERIC  .     312 

vii 


viii  Contents 

CHAPTER  PAGE 

XI. — THE  ATTACK  AND    DEFENSE    OF    THE 

CONTINENTS         ....     335 

XII. — THE    OLD  AGE   AND   DEATH   OF   THE 

EARTH 356 

INDEX 367 


ILLUSTRATIONS 

FIG.  PAGE 

1.  GREAT  NEBULA  IN  ORION         .         .       10 

2.  SPIRAL    NEBULA    IN    THE    CONSTEL- 

LATION OF  CANES  VENATICI        .       10 

3.  PLANETARY  NEBULA  IN  LYRA  .         .       n 

4.  DIAGRAM  OF  THE  TERRESTRIAL  "TE- 

TRAHEDRON" 32 

5A.    LIPPMANN'S     HYPOTHESIS     ON     THE 

EARTH'S  CRUST  54 

SB.   LIPPMANN'S   HYPOTHESIS  ...       54 

6.  THE  LAND  HEMISPHERE  AND  WATER 

HEMISPHERE,  TAKING  ILE  DUMET 

AS  POLAR  POINT        w         .         .       67 

7.  DEVIATION  OF  A  PLUMB-LINE  BY  A 

MOUNTAIN         .  .         -77 

8.  CAVENDISH'S  EXPERIMENT         .         .       82 

9.  LATITUDE  AND  CO-LATITUDE      '.  .      .     113 
10.     FLUCTUATION  OF  LATITUDES     .         .114 


x  Illustrations 

FIG.  PAGE 

11.  DISPLACEMENT  OF  THE  NORTH  POLE 

ON  THE  EARTH'S  SURFACE.  (One 
side  of  the  square  represents  a  length 
of  about  20  metres)  .  .  .116 

12.  PRECISION  OF  PENDULUM  MEASURES    151 

13.  THE  FORM  IMPRESSED  ON  THE  EARTH 

BY  THE  COMBINED  ACTION  OF 
GRAVITY  AND  CENTRIFUGAL  FORCE  1 53 

14-17.  DIFFERENT  FORMS  OF  THE  THEO- 
RETICAL CURVE  DESCRIBED  BY 
THE  PLUMMET  OF  A  PLUMB-LINE 
ACCORDING  TO  THE  VALUES  OF 
THE  MOON'S  DECLINATION.  .  177 

1 8.     PRINCIPLE  OF  HORIZONTAL  PENDULUM     184 

19-20.     ACTUAL  CURVES  DESCRIBED  BY  THE 

EXTREMITY  OF  A  PLUMB-LINE    .     187 

2 1 .  DEVIATION  OF  THE  SOUTHERN  PORTIONS 

OF  THE  CONTINENTS  TOWARDS 
THE  EAST  (HECKER)  .  .214 

22.  DISTRIBUTION  OF  VOLCANOES.    INTER- 

CONTINENTAL DEPRESSION  .     215 

23.  MAGNETIC  FIGURE   (Two   POLES   OF 

CONTRARY  SIGN),  AFTER  PRO- 
FESSOR STANO'IEVITCH  ,  .  234 


Ill-ustrations  xi 

FIG.  PAGE 

24.  MAGNETIC   FIGURE   (Two  POLES  OF 

A  CIRCULAR  MAGNET),  AFTER 
PROFESSOR  STANOIEVITCH  .  .  234 

25.  SECULAR    VARIATIONS    OF   THE    DE- 

CLINATION AT  PARIS  .         .         .238 

26.  MAP  SHOWING  ISOGONIC  LINES,  I.E., 

LINES  OF  EQUAL  MAGNETIC  DE- 
CLINATION ....  245 

27.  LIQUID  PROTUBERANCES  PRODUCED  ON 

THE  WATERS  OF  THE  SEA  BY  THE 
ATTRACTION  OF  A  NEIGHBOURING 
BODY 284 

28.  TRACING    A    SINE-CURVE   BY   MEANS 

OF  A  ROD  AND   CRANK   SYSTEM    295 

29.  PRINCIPLE  OF  THE  TIDE   PREDICTER; 

COMBINATION  OF  Six  ELEMENTARY 
WAVES  .....  296 

30.  CIRCULAR  VIBRATORY  MOVEMENTS  OF 

MOLECULES  OF  A  LIQUID.  FOR- 
MATION AND  PROPAGATION  OF  THE 
SWELL  .  •'";  '  .  _  .  .  307 


The  Earth:  Its  Life  and  Death 


CHAPTER  I 

THE  BIRTH  OF  THE  EARTH 

TTOW  did  the  little  globe  on  which  we  live  come 
*  A  into  existence  in  the  midst  of  the  universe? 
Under  what  circumstances  did  it  originate?  What 
were  the  causes  which  determined  its  constitu- 
tion, its  primaeval  form?  In  other  words,  what 
were  the  conditions  of  its  conception  and  its  birth? 
These  questions,  which  arise  naturally  at  the 
beginning  of  this  work,  must  always  have  been 
present  to  the  mind  of  man.  But  it  is  only  com- 
paratively recently  that  sufficient  light  has  been 
thrown  upon  them,  if  not  to  solve  them  com- 
pletely, at  any  rate  to  enable  us  to  obtain  some 
idea  of  the  forces  which  were  at  work,  and  the 
conditions  which  obtained,  during  the  inception 
of  our  Earth,  and  during  its  formation  as  an 
individual  world  in  the  midst  of  space. 


*    %**  „    V      *   *j?  *  *  ,  c 

1  '    *   *    '>     ?» 

•i>?'  THe  EartK 

If,  on  a  clear  dark  night,  we  look  upwards  at 
that  expanse  which  we  call  the  sky,  and  which 
seems  to  be  a  great  dome  supported  upon  the 
horizon,  we  see  there  a  wonderful  spectacle, 
which  appears,  at  the  first  glance,  absolutely  in- 
extricable. Attentive  observation  of  the  heavens 
has,  however,  enabled  astronomers  to  classify  the 
phenomena  in  evidence,  and,  at  the  same  time, 
to  simplify  the  difficult  problem  which  constitutes 
their  study. 

In  the  first  place,  among  the  innumerable  bril- 
liant points  which  stud  the  sky,  there  are  some 
which  scintillate  and  others  which  shine  with  a 
steady  light.  However  great  the  magnification 
secured  by  the  telescope  utilised,  the  former  ap- 
pear only  as  geometrical1  points;  these  are  the 
stars  which  seem  to  rotate  with  a  continuous 
movement  about  an  imaginary  line  passing  through 
a  definite  point  in  the  heavens.  On  the  other 
hand,  the  latter,  which  are  the  planets,  appear 
larger  with  each  increase  of  power  of  the  instru- 
ments that  we  use  to  study  them.  Their  move- 
ments differ  from  those  of  the  stars;  they 

1  Theoretically  this  is  so,  but,  actually,  a  star  viewed  through  a 
telescope  shows  a  disk.  This  is  called  "spurious,"  as  it  appears 
merely  because  of  a  certain  lack  of  perfection  in  all  lenses.  This 
spurious  disk  does  not  increase  in  size  when  higher  magnifying 
powers  are  used. — Ed. 


The  DirtH  of  tHe  EartH  3 

revolve  around  a  great  and  brilliant  globe,  the 
Sun. 

The  Sun  itself  appears  to  revolve  about  us, 
bringing  the  day  when  it  is  visible  and  leaving  us 
to  night  when  it  disappears  beneath  the  horizon, 
the  truth  being,  however,  that  it  is  actually  the 
Earth  which  moves,  turning  about  its  own  axis. 

A  second  globe  which  shines  with  brilliancy  only 
during  the  night,  and  then  only  at  certain  times  and 
under  variable  aspects,  is  more  immediately  de- 
pendent upon  the  Earth  around  which  it  revolves; 
this  is  the  Moon. 

Other  phenomena,  also,  .are  exhibited  by  the 
celestial  bodies.  In  some  places,  a  large  number 
of  very  small  stars  are  gathered  together  so  that, 
while  they  are  all  distinct  from  one  another,  they 
form  a  kind  of  luminous  spot  which  is  called  a 
star-cluster;  in  other  places  are  seen  regions  shin- 
ing with  a  milky  luminosity,  distinct  from  the 
background  of  dark  sky,  having  various  forms, 
indefinite,  spiral,  or  circular.  These  are  the 
nebulae.  An  immense  whitish  track  of  light  trav- 
erses and  encircles  the  entire  sky;  this  is  called 
the  Milky  Way  and  is  an  aggregation  of  a  very 
great  number  of  little  stars,  of  which  our  Sun 
doubtless  forms  one,  gigantic  for  us,  microscopic 
as  compared  to  the  immensity  of  space. 


4  The  Earth 

Transitory  bodies  appear  suddenly  in  the  heav- 
ens; these  are  the  meteors  which  resemble  fire- 
works, and  which  come  often  in  considerable 
numbers  and  at  fixed  epochs;  they  are  called 
"shooting  stars."  At  certain  times,  special  bodies 
appear  with  a  luminous  nucleus  and  a  long  tail. 
These  are  comets  and  their  periods  are  often  very 
long.  Occasionally,  new  stars  blaze  up  suddenly 
and  take  on  temporarily  the  appearance  of  the 
fixed  stars.  Their  brilliancy  increases,  remains 
for  a  time  stationary,  and  then  gradually  decreases 
prior  to  their  disappearance. 

These  are  the  simplest  observational  facts  that 
astronomers  have  learned.  If  we  now  examine 
the  heavenly  bodies,  not  with  an  ordinary  tele- 
scope, but  with  that  wonderful  apparatus  known 
as  the  spectroscope,  which,  by  splitting  up  light 
into  its  component  rays,  enables  us  to  analyse  it 
with  great  precision,  we  are  able  to  prove  that,  at 
any  rate,  as  far  as  the  Sun  and  the  Earth  are  con- 
cerned, they  are  composed,  for  the  most  part,  of 
the  same  chemical  elements.  This  fact,  alone, 
is  suggestive  of  their  having  had  a  common  origin. 
Some  elements  which,  formerly,  were  known  to 
exist  only  in  the  Sun  have  since  been  discovered 
in  the  matter  of  which  our  Earth  is  formed. 

Our  globe,  by  the  character  of  its  movements, 


The  DirtH  of  the  EartK  5 

is  analogous  to  the  planets,  which  are  all  of  a  sphe- 
roidal form  and  are  isolated  in  space,  with  a  move- 
ment of  revolution  around  the  Sun.  Now,  voyages 
of  circumnavigation,  in  the  course  of  which  the 
sailors  have  made  the  entire  journey  round  the 
Earth,  demonstrate  to  us  that  it  also  is  isolated 
in  space.  Again,  the  circular  form  of  the  shadow 
which  it  throws  upon  the  Moon  during  the  eclipses 
of  that  body  proves  its  globular  form.  There  is, 
therefore,  one  conclusion  that  we  can  draw  from 
our  preliminary  study.  The  Earth  is  a  planet 
belonging  to  the  Solar  System,  constituted  of  the 
same  elements  as  the  central  Sun  of  that  System. 
Another  piece  of  information  that  the  spectro- 
scope affords  us  is  that  the  Sun  and  the  stars  are 
self-luminous  bodies,  while  the  planets  and  the 
Moon  are  illuminated  bodies  which  appear  bright 
to  us  because  they  reflect  the  Sun's  light  and  not 
because  they  emit  light  themselves.  The  self- 
luminous  stars  and  the  Sun  must  therefore  be  at 
a  very  high  temperature,  in  the  midst  of  celestial 
space,  which  the  investigations  of  astrophysicists 
have  shown  to  be  very  cold.  Just  as  when  the 
colour  of  a  piece  of  iron  heated  in  a  forge  passes, 
gradually  as  it  cools,  from  dazzling  white  to  yellow, 
then  to  orange,  to  bright  red,  and  lastly  to  dull 
red,  we  can,  from  the  colour,  deduce  the  tempera- 


6  TKe  Earth 

$ 

ture  of  the  iron,  so  from  the  colour  of  a  star  we 
can  obtain  information  as  to  its  temperature  and 
age.  The  blue  and  white  stars  are  in  the  height 
of  their  period  of  incandescence.  The  yellow 
stars  have  already  cooled  to  some  extent;  this  is 
the  condition  of  our  Sun.  The  yellow  and  orange 
stars  are  already  on  the  way  towards  the  period  of 
a  more  advanced  cooling  which,  after  the  red  phase, 
leads  to  the  solidification  of  their  surfaces,  ren- 
dering these  obscure  and  transforming  them  into 
extinct  suns. 

Among  the  facts  observed  in  the  sky  and  which 
seem  to  overwhelm  the  reason,  we  must  place  first 
that  admirable  mechanism  which  maintains  the 
bodies  isolated  in  space  and  which  causes  some  of 
them  to  revolve  around  others. 

This  wonderful  adjustment  was  a  mystery  to 
man,  until  Kepler  formulated  the  laws  under 
which  the  planetary  orbits  are  described  and 
Newton  discovered  and  enunciated  the  law  of 
universal  gravitation,  from  which  Kepler's  laws 
can  be  deduced  mathematically. 

Kepler  found  that  the  planetary  orbits  are  not 
exactly  circular,  but  are  ellipses  of  which  the  Sun 
occupies  one  of  the  foci.  This  is  the  first  law, 
from  which  it  follows  that  the  distance  from  the 


THe  DirtH  of  the  EartK  7 

centre  of  the  planet  to  the  centre  of  the  Sun,  the 
radius  vector,  to  give  it  its  mathematical  name,  is 
not  a  fixed  length  but  varies  according  to  the 
position  of  the  body  in  its  orbit.  Kepler's  second 
law  deals  with  this  variation  in  the  length  of  the 
radius  vector;  it  states  that  the  area  swept  over 
by  the  radius  vector  in  any  given  time  is  always 
the  same,  whatever  the  position  of  the  planet  in 
its  orbit.  This  law  governs  the  velocity  with 
which  a  planet  pursues  its  path;  it  moves  more 
rapidly  in  proportion  as  it  is  nearer  the  Sun,  and 
more  slowly  in  proportion  as  it  is  farther  from 
the  Sun.  The  third  law  governs  the  time  that  the 
planets  take  to  traverse  their  orbits  entirely;  the 
square  of  the  period  is  proportional  to  the  cube 
of  the  major  axis  of  the  elliptical  orbit  in  the  case 
of  each  planet. 

From  these  laws  Newton  was  able  to  read  some- 
thing more  than  the  principles  of  the  movements 
that  they  enunciated.  He  divined  from  them  the 
cause  of  these  planetary  movements  and  he  gave 
the  great  and  simple  formula  determining  these 
in  his  law  of  attraction  or  universal  gravitation. 

"Any  two  bodies  attract  each  other  with  a  force 
directly  proportional  to  the  product  of  their 
masses  and  inversely  proportional  to  the  square 
of  their  distance  apart." 


8  THe  EartH 

This  law  in  giving  the  principle  of  force  gives 
also  that  of  the  movement  produced  by  the  force. 
It  can  be  demonstrated  by  mechanics  that  Kepler's 
laws  can  be  rigorously  deduced  from  the  law  of 
gravitation. 

We  now  know  another  fundamental  law,  the 
existence  and  the  value  of  the  pressure  of  radia- 
tion. It  is  by  radiation,  of  which  luminous  radia- 
tion is  the  most  obvious  kind,  that  we  are  aware 
of  the  existence  of  the  distant  worlds  with  which 
infinite  space  is  strewn.  Apart  from  the  luminous 
radiations  perceived  by  the  retina  of  the  eye,  there 
are  other  kinds  which  that  organ  is  incapable  of 
distinguishing.  If  a  ray  of  sunlight  be  passed 
through  a  prism  of  quartz,  and  if  the  polychro- 
matic image  that  is  thus  observed,  and  which  is 
called  a  spectrum,  be  photographed  it  is  found  that 
the  photographic  image  is  prolonged  some  way 
beyond  the  extreme  violet  perceptible  to  the  eye. 
There  are  thus  ultra-violet  rays  of  which  our 
senses  cannot  inform  us,  but  which  the  photo- 
graphic plate  registers.  A  very  sensitive  ther- 
mometer, placed  in  the  region  which  precedes 
the  red  rays,  discloses  there  infra-red  rays  imper- 
ceptible to  the  sight. 

These  radiations,  to  which  must  be  added  electric 
radiations  and  other  forms  that  will  probably  be 


The  Birth  of  tHe  EartH  9 

discovered  in  the  future,  are  the  means  whereby 
the  forces  of  the  universe  are  transmitted.  The 
great  physicist  Maxwell  first  discovered,  in  1873, 
that  radiation  exerts  a  veritable  pressure,  the 
power  of  which  is  measured  by  the  quantity  of 
energy  contained  in  a  unit  volume  of  the  medium 
transmitting  it.  Maxwell  feared  that  the  small- 
ness  of  this  pressure  might  render  its  measure 
impossible,  but  to  the  Russian  physicist  Labedeff 
is  due  the  credit  of  achieving  this  difficult  measure- 
ment. If  we  imagine  a  black  body  placed  against 
the  Sun's  surface,  the  radiation  emitted  by  this 
surface  exercises  a  pressure  of  2.75  milligrammes 
[.042  gr.]  per  square  centimetre  [.155  sq.  in.]  on 
the  body.  The  illustrious  Swedish  physicist 
Arrhenius,  to  whom  science  owes  a  new  outlook 
upon  the  formation  of  worlds,  has  been  able  to 
demonstrate  mathematically  that  for  a  very  small 
sphere,  which  is  not  transparent  and  of  which  the 
diameter  is  somewhat  less  than  a  micron,  that  is 
to  say  is  less  than  a  thousandth  of  a  millimetre 
[.00003937  in.],  situated  in  the  neighbourhood  of 
the  Sun,  the  repulsive  force  resulting  from  the 
pressure  of  radiation  would  be  greater  than  the 
attraction  of  the  Sun's  mass,  and  so  the  small 
body  would  be  driven  far  away  into  space.  If 
the  diameter  of  the  small  sphere  be  supposed  to 


10  The  EartH 

be  less,  assuming  its  density  to  remain  equal  to  I, 
the  repulsive  force  would  be  increased,  but  such 
increase  could  not  continue  indefinitely,  for,  if  the 
particle  were  much  smaller  than  a  wave-length 
of  the  light  acting  upon  it,  diffraction  phenomena 
would  be  produced  which  would  completely  alter 
the  nature  of  the  light  action.  For  particles 
having  a  diameter  of  .00015  millimetre,  the  repul- 
sive force  is  ten  times  greater  than  the  attractive 
one  due  to  gravity. 

The  self-luminous  bodies,  the  Sun  and  stars, 
thus  have  the  power  of  driving  away  into  space 
those  very  small  particles  of  matter  of  low  density, 
and  these  particles  constitute  the  cosmic  dust  that 
permeates  interstellar  space.  Particles  similarly 
repulsed  constitute  the  tails  of  comets,  which 
are  always  directed  away  from  the  Sun,  as  if 
under  the  action  of  wind  from  the  latter  body. 
In  all  probability,  the  Solar  Corona  itself  is  com- 
posed of  such  minute  fragments  of  matter.  The 
dust  particles  that  are  thus  repelled  and  flung 
into  space  by  the  stars  are  negatively  electrified, 
the  star  remaining  positively  charged.  Such  of 
those  emanating  from  the  Sun,  which  reach  the 
Earth,  produce  by  their  negative  charges  important 
electric  effects,  as  we  shall  see  later  on. 

In  addition  to  the  suns  which  shine  in  the 


FIG.  i . — Great  Nebula  in  Orion. 


FIG.  2.— Spiral  Nebula  in  the  Constellation  of 
Canes  Venatiei. 


FIG.  3. — Planetary  Nebula  in  Lyra. 


THe  BirtH  of  tKe  Earth  n 

heavens  and  which  are  bodies  radiating  heat, 
there  are  cold  bodies  in  interstellar  space  whose 
useful  purpose  seems  to  be  to  arrest  and  absorb 
the  heat  energy  radiated  by  the  suns.  If  these 
did  not  exist,  the  infinity  of  stars  scattered  through- 
out space  would  give  to  the  sky  the  aspect  of 
a  fiery  vault,  an  incandescent  dome,  and  their 
aggregate  radiation  would  annihilate  all  manifesta- 
tion of  life  at  the  surface  of  the  habitable  globes. 
These  cold  bodies  are  the  nebulae1,  they  exist 
everywhere  scattered  through  space.  [However, 
where  the  stars  are  most  numerous,  in  the  Milky 
Way,  the  visible  nebulae  are  very  scarce,  and 
conversely  the  nebulae  abound  where  the  stars  are 
fewest. — Ed.]  They  can  be  seen  in  the  clear  dark 
sky  as  milky  patches  of  light,2  some  without  de- 
finite form  or  contour  (Fig.  i),  others  with  a  roughly 
circular  shape  in  which  can  almost  always  be 
traced  a  tendency  to  form  a  more  or  less  sharply 
materialised  spiral.  [This  form,  in  fact,  predomi- 
nates to  a  great  degree. — Ed.]  In  some  further 
cases  the  contour  is  still  more  definite.  These 


1  While  scientists  are  almost  unanimous  in  agreeing  that  some 
light-  and  heat-absorbing  medium  does  exist,  it  is  not  as  gener- 
ally accepted  that  nebulae  exist  in  sufficient  numbers  to  be  this 
medium. — Ed. 

1  Very  few  are  visible  without  optical  aid,  only  two  in  the 
northern  sky. — Trans. 


12  TKe  Earth 

are  the  "planetary"  nebulas1  which  consist  of  a 
central  nucleus  surrounded  symmetrically  by  an 
"atmosphere  of  light."  Spectrum  analysis  en- 
ables us  to  study  the  composition  of  the  nebulas ; 
their  spectra  are  composed  of  bright  lines  similar 
to  those  which  are  obtained  in  our  laboratories 
from  incandescent  gases.  [This  refers  only  to 
the  so-called  green  nebulas.  The  predominating 
type,  the  spiral  nebulas,  known  spectroscopically 
as  white  nebulas,  present  spectra  radically  differ- 
ent.— Ed.]  The  characteristic  rays  given  by 
hydrogen  and  by  helium,  a  gaseous  element  orig- 
inally discovered  in  the  Sun  and  recently  [1895. 
— Ed.]  extracted  from  terrestrial  sources,  can  be 
recognised,  also  rays  from  an  unknown  substance, 
not  yet  found  in  the  Earth,  to  which  the  name 
nebulium  has  been  assigned.  Helium  is  formed 
by  the  molecular  disintegration  of  the  radioactive 
substances  which  exist  in  the  solid  crusts  of  the 
planets ;  possibly  all  the  constituents  of  these,  that 
is,  all  forms  of  matter,  are  radioactive.  Both 
hydrogen  and  helium  diffuse  into  space,  where 
their  thinly  scattered  molecules  constitute  the 
rarefied  nebulous  medium  which  only  requires  the 
coming  of  a  nucleus  for  condensation  to  commence 

1  So  named  by  Sir  William  Herschel  because  of  their  appear- 
ance, which  is  that  of  an  ill-defined  and  hazy  disk. — Ed. 


I 
TKe  BirtH  of  the  Earth  13 

there.  Helium  and  hydrogen,  therefore,  might 
appear  to  be  the  ultimate  products  of  the  disin- 
tegration of  matter. x 

Whatever  degree  of  precision  the  outlines  of  the 
spiral  nebulae  show,  streams  of  matter  appear  to 
diverge  from  a  central  point  and  to  become  more 
or  less  blended  as  they  spread  out,  thus  indicating 
a  general  movement  of  rotation  of  the  matter 
composing  the  nebula.  [The  rotation  of  a  spiral 
nebula  was  proved,  in  the  spring  of  1914,  by  spec- 
trum photographs  made  at  the  Lowell  Observatory. 
— Ed.]  But  one  of  the  chief  features  of  these  ob- 
jects is  the  existence  in  them  of  nuclei,  more  bril- 
liant than  the  remaining  part,  apparently  centres 
of  condensation,  around  which  the  nebulous 
matter  accumulates  as  it  becomes  more  dense, 
thus  giving  birth  to  stars.  The  great  spiral  nebula 
in  Canes  Venatici  is  a  remarkable  example,  in 
which  there  is  also  a  second  nucleus  apart  from 
the  principal  one  (Fig.  2). 

How  do "  these  condensation  centres  arise? 
Three  explanations  have  been  advanced.  First, 
the  contraction  due  to  cooling  through  many  ages 
suffices  to  increase  the  density  at  the  centre,  in 
consequence  of  which  the  velocity  of  rotation,  and, 

1  Recent  investigations  have  seemed  to  show  the  possibility 
of  the  transformation  of  hydrogen  into  helium. — Trans. 


14  THe  Earth 

therefore,  also  the  resulting  centrifugal  force,  is 
augmented,  thus  causing  the  detachment  of  annu- 
lar masses  from  the  further  parts  of  the  nebula. 
This  is  what  scientists,  following  Laplace,  thought 
at  the  beginning  of  the  nineteenth  century.  [It  is 
now  known,  however,  that  nebulae  cannot  be  at 
a  high  temperature. — Ed.]  Secondly,  there  is  the 
explanation  of  modern  astrophysicists,  following 
Arrhenius,  viz.:  that,  in  the  course  of  those  in- 
numerable centuries  which  are  only  an  instant  in 
the  eternal  history  of  worlds,  "dead  suns"  enter 
the  nebula  and  serve  as  nuclei  for  condensation. 
Thirdly,  bodies,  such  as  the  Earth  is  now  and  as 
the  Sun  will  be  in  the  future,  superficially  cooled, 
but  nevertheless  containing  in  their  heated  internal 
masses  a  tremendous  reserve  of  energy,  have  given 
birth,  by  collision  with  each  other  and  the  conse- 
quent partial  volatilisation  due  to  the  heat  disen- 
gaged by  the  shock,  to  a  nebula  for  which  the 
remaining  portions  of  the  original  bodies  serve  as 
a  nucleus,  thus  giving  rise  to  a  new  star  and  so 
exemplifying  the  resurrection  of  a  world.  [The 
collision,  or  even  very  close  approach,  of  two  large 
absolutely  cold  bodies  would  produce  practically 
the  same  result. — Ed.] 

Whatever  the  original  cause,  the  fact  is  almost 
certainly  established  that  stars,  that  is  to  say, 


The  DirtK  of  tHe  EartH  15 

suns,  are  produced  from  nebulae  by  condensation 
of  the  matter  forming  these.  When  a  fragment  of 
cosmic  dust  penetrates  into  the  midst  of  the  nebu- 
las, it  falls  towards  the  centre  of  gravity  of  the 
whole,  and  the  more  the  condensation  progresses 
the  greater  the  rise  of  temperature.  The  imper- 
ishable fame  of  Laplace  lies  in  his  having  been  the 
first  to  indicate  the  way  in  which  our  Solar  System 
was  derived  from  its  original  nebula. 

Nebulae,  in  their  early  stages,  are  composed  of 
gases  in  a  state  of  extreme  rarefaction,  of  which 
the  contents  of  a  Crookes  tube  give  us  some  idea. 
They  retain  what  reaches  them  of  the  cosmic  dust 
thrust  outwards  from  the  suns  by  the  pressure  of 
radiation.  The  nebulae  at  first  have  the  properties 
of  gaseous  masses  in  adiabatic  equilibrium;  that 
is  to  say,  when  they  receive  heat  from  neighbouring 
suns  their  temperature  does  not  rise,  but  falls. 
Thus,  according  to  Arrhenius,  they  have  a  nega- 
tive specific  heat.  Since  the  cosmic  dust  is  electri- 
fied a  charge  accumulates  in  the  outer  layers  of 
the  rarefied  gaseous  mass.  It  should  be  noticed 
that  the  temperature  of  a  nebula  must  be  very- 
low,  on  account  of  its  rarefaction,  which  implies 
an  absence  of  internal  movements  and  therefore 
of  molecular  collisions  giving  rise  to  heat.  In  all 
probability,  the  temperature  of  such  a  nebula  is 


16  THe  EartH 

about  50°  C.  [90°  F.]  above  the  absolute  zero  of 
the  physicists,  which  zero  has  been  definitely 
established  by  Amagat  to  be  273°C.  [491. 4°F.] 
below  the  temperature  of  melting  ice,  this  latter 
being  the  zero  of  our  thermometer  [centigrade] 
for  practical  purposes.  It  is  this  recognition  of 
the  low  initial  temperature  of  the  nebula  which 
constitutes  one  of  the  most  essential  modifica- 
tions that  modern  science  has  made  in  Laplace's 
theory  of  the  evolution  of  the  Solar  System.  He 
assumed  that  the  nebula  was  originally  at  a  high 
temperature. 

Now,  in  spite  of  the  low  temperature  we  have 
been  led  to  attribute  to  it,  the  nebulous  mass 
emits  light,  and  so  is  visible  to  us  by  reason  of  the 
luminosity  with  which  its  constituent  materials 
shine.  It  seems  remarkable  how  this  state  of 
incandescence  can  be  maintained  in  these  cir- 
cumstances. It  is  due  to  the  fact  that,  in  propor- 
tion as  the  accumulating,  electrified  oust  particles 
add  an  increasing  quantity  of  electricity  to  the 
periphery  of  the  nebula,  the  strain  increases  little 
by  little,  and  ends  by  becoming  sufficient  for  a 
discharge,  analogous  to  that  which  takes  place  in 
a  Crookes  tube.  This  illuminates  the  entire 
mass,  thus  rendering  it  visible  against  the  dark 
background  of  the  sky.  It  should,  therefore,  be 


THe  Birth  of  tKe  EartK  17 

noticed  that  we  cannot  see  any  such  nebula  in 
which  the  electric  stress  is  not  yet  great  enough 
to  have  produced  discharge  luminosity  in  the 
gaseous  masses  which  compose  its  outer  layers, 
so  that  the  number  of  known  nebulae  must  be 
vastly,  perhaps  almost  infinitely,  increased  if  this 
number  is  to  represent  all  that  actually  exist.  In 
all  cases,  the  production  of  luminosity  is  the  first 
stage  in  the  life  of  a  nebula,  hitherto,  figuratively 
speaking,  inert. 

The  second  stage  is  the  formation  of  a  nucleus. 
Possibly  a  cooled  body  like  the  Moon  or  Earth 
comes  in  the  course  of  ages  and  penetrates  into 
the  nebula,  or  denser  masses  of  particles  ag- 
glomerated together  as  meteorites  similarly  enter. 
Or  perhaps  the  moving  gaseous  molecules  collect 
in  one  part  from  some  cause.  Condensation  at 
once  begins  around  these  intruded  masses,  as 
they  may  be  called,  which  are  therefore  the  means 
of  starting  the  condensation  process.  This  pro- 
cess sets  heat  free,  and  the  nucleus,  which  grows 
continuously,  gradually  becomes  incandescent, 
after  having  captured  the  greater  part  of  the  rare- 
fied matter  which  constituted  the  original  nebula. 
The  system  has  now  reached  the  stellar  phase. 
As  the  condensation  continues,  the  pressure  at 
the  centre  increases  and  soon  becomes  very  large. 


1 8  The  Earth 

The  original  hydrogen  and  helium,  the  residue 
of  the  disintegration  of  the  matter  of  other  stars, 
now  become  the  origins  of,  or  points  of  departure 
for,  the  integration  of  the  matter  of  a  new  star. 

It  may  also  happen,  as  has  been  said  above, 
that  two  dead  suns  clash  together  in  the  course 
of  their  journeyings  through  space,  at  some  time 
or  other.  If  they  are  constituted  similarly  to  the 
Earth,  their  frail  envelopes  would  be  broken  by  the 
force  of  the  shock  and,  independently  of  the  enor- 
mous amount  of  heat  disengaged  by  the  impact, 
the  fiery  material  set  free  by  the  rupture  of  the 
containing  crust  would  rush  out  into  space,  being, 
for  the  most  part,  volatilised  owing  to  the  sudden 
decrease  of  pressure.1  Two  jets  of  spiral  form 
would  be  produced,  the  whole  rotating  by  reason 
of  the  obliquity  of  the  impact  of  the  two  bodies. 
A  new  nebula  may  thus  be  created  out  of  two  dead 
suns.  At  its  centre  would  be  a  new  sun,  or  per- 
haps two  or  even  more  suns.  This  is  an  explana- 
tion of  the  appearance  of  those  novae  or  new  stars 
which  so  strongly  arouse  the  interest  of  astrono- 
mers. In  all  observed  cases  a  spiral  nebula  has 
come  into  being  with  one  or  several  incandescent 

1  The  heat  generated  by  the  impact  and  the  tidal  strain  in  each 
body  would  be  of  far  greater  effect  than  the  release  of  the  heat 
existing  in  the  inner  parts  of  the  two  bodies;  these  in  fact  might 
be  cold  to  the  centre  and  yet  be  disrupted  and  volatilised. — Ed. 


The  BirtK  of  tHe  EartK  19 

nuclei.  Minor  centres  of  condensation,  arising 
perhaps  from  masses  thrown  out  during  the 
original  collision,  are  found  in  the  spirals.  These 
secondary  suns  originate  immediately  after  the 
primary  one,  drawing  to  them  a  part  of  the  en- 
compassing cosmic  material ;  they  gravitate  around 
the  central  and  more  important  sun,  and  so  a 
planetary  system  is  given  birth. 

The  planets,  at  the  commencement  of  their  his- 
tory, are  formed  of  practically  the  same  elements 
as  the  central  nucleus.  Carried  round  by  the 
initial  movement  of  rotation,  they  all  generally 
revolve  the  same  way,  except  in  the  cases  where 
a  strange  body  previously  rotating  in  a  contrary 
way  may  have  penetrated  into  the  exterior  limits 
of  the  nebula,  and  so  come  into  the  field  of  attrac- 
tion exercised  by  the  new  sun,  the  satellites  of  this 
body  retaining  their  primitive  rotatory  sense  on 
account  of  its  mass.  This  is  perhaps  what  occurred 
with  regard  to  the  outermost  planets  of  our  Solar 
System,  Uranus  and  Neptune.1  It  is  not  neces- 
sary to  assume  that  adventitious  bodies  entered 
the  nebula  to  provide  planets  for  the  primitive 
central  body.  Laplace  held  that  the  centrifugal 


1  Other  interesting  and  very  plausible  theories  are  propounded 
for  the  explanation  of  the  rotations  of  Uranus,  Neptune,  and  their 
satellites.— Ed. 


20  The  EartH 

force  would  suffice  to  detach  successive  equatorial 
rings  from  the  mass  of  the  principal  nucleus,  the 
speed  of  rotation  of  which  increases  in  proportion 
as  it  contracts  by  cooling ;  that  each  of  these  rings 
afterwards  would  become  a  planet  by  the  process 
of  agglomeration  of  its  material  at  one  point ;  and 
that,  in  its  turn,  the  planet  might  produce  one  or 
more  satellites  by  an  exactly  similar  method. 
[That  the  planets  and  their  satellites  could  have 
been,  and  probably  were,  all  built  from  the  parent 
body  or  bodies  and  their  particles  and  gases  is 
accepted  by  the  scientist  of  to-day.  The  equa- 
torial ring  theory  of  Laplace,  is,  however,  no  longer 
credited.— Ed] 

We  have  now,  therefore,  attained  to  the  concep- 
tion of  a  nebula  having  a  principal  centre  of  con- 
densation, that  is  to  say  having  a  central  sun  and 
also  secondary  centres,  whether  formed  by  the 
intrusion  of  adventitious  bodies  or  arising  from 
the  condensations  and  agglomerations  of  matter 
detached  from  the  principal  mass  first,  possibly 
by  the  initial  catastrophe  or  later  by  the  agency 
of  centrifugal  force.  The  subsidiary  centres  begin 
to  gravitate  around  the  chief  one,  describing  ellip- 
tical orbits,  the  form  of  which  was  defined  by 
Kepler,  who  also  first  stated  the  laws  of  the 
planetary  movements.  The  secondary  nuclei 


THe  BirtH  of  tHe  Earth  21 

begin  to  rotate  on  their  axes  in  consequence  of 
the  initial  movement  of  rotation  of  the  primitive 
nebula.  We  shall  here  only  consider  one  of 
these  bodies,  the  Earth. 

From  the  time  when  it  was  separated  from  the 
central  nebulous  mass,  the  Earth's  individuality 
commenced,  but  it  had  not  yet  become  what  may 
be  called  the  terrestrial  globe.  Before  doing  so, 
it  had  to  cool  and  consequently  to  contract.  It 
can  be  shown  by  mechanics  that  the  velocity  of 
rotation  increased  as  the  diameter  diminished. 
Centrifugal  force  caused  a  mass  of  matter  to  be 
detached  from  the  Earth's  equator,  the  Earth 
having  been  previously  flattened  to  the  shape  of 
an  orange  by  the  same  agency,  [and  later,  just 
before  the  mass  broke  loose,  drawn  out  into  a 
somewhat  pear-shaped  form. — Ed.]  This  de- 
tached body  took  the  normal  spheroidal  form 
round  a  nucleus,  the  small  mass  of  which  per- 
mitted a  more  rapid  cooling.  Thus,  the  Moon 
was  formed  and  has  subsequently  continued  its 
revolution  about  the  Earth. 

The  temperature  of  the  detached  Earth  fell 
much  more  quickly  than  that  of  the  central  Sun, 
which,  on  account  of  its  enormous  mass,  325,000 
times  that  of  the  Earth,  cooled  with  extreme  slow- 
ness, just  as  of  two  pieces  of  iron  heated  red  hot 


22  TKe  EartK 

in  the  same  fire  the  greater  remains  warm  a  much 
longer  time  than  the  smaller.  The  mass  constitut- 
ing the  Earth,  therefore,  passed  gradually  from 
the  gaseous  to  the  liquid  state,  and  then  to  a  vis- 
cous condition.  Its  rotation,  which  implied  a 
concomitant  centrifugal  force,  then  caused  the 
equator  to  bulge  out  and,  also,  the  polar  regions 
to  become  flattened.  [It  was  succeeding  this  that 
the  Moon  was  cast  off. — Ed.]  In  proportion  to 
the  extent  of  the  cooling,  some  of  the  gaseous 
elements  which  constituted  its  atmosphere,  dis- 
sociated and  kept  from  combination  by  the  high 
initial  temperature,  were  enabled  to  condense,  as, 
for  example,  metals  that  were  originally  vaporised, 
and  others  to  combine  together  when  they  arrived 
at  a  sufficiently  low  temperature.  As  the  cooling 
steadily  proceeded  during  this  time,  the  globe 
came  to  solidify  at  its  exterior  surface,  and  there- 
fore to  be  covered  with  a  crust  which,  although 
very  thin  at  first,  gradually  thickened  until  it 
served  to  maintain  a  kind  of  equilibrium  between 
the  escaping  internal  heat,  which  it  transmitted 
badly,  on  account  of  its  feeble  conductivity,  and 
the  external  heat  received  from  the  central  Sun. 

Thus,  we  have  a  globe,  flattened  at  the  poles 
and  protruding  at  the  equator,  covered  by  a  solid 
crust,  and  of  which  the  chief  part  consists  of  in- 


THe  BirtK  of  tHe  EartH  23 

candescent  material  at  a  highly  elevated  tempera- 
ture. The  crust  is  encompassed  by  an  atmosphere 
in  which  were  originally  present  the  vapours  of 
all  substances  volatile  at  the  temperature  of  solidi- 
fication of  the  materials  which  constituted  the 
solid  crust.  The  Earth  has  come  into  being. 


CHAPTER  II 

THE  AGE  OF  THE  EARTH 

\  A  fE  have  now  to  consider  the  first  stage  of  the 
*  *  existence  of  the  Earth,  the  origin  of  which 
was  characterised  by  the  formation  of  a  superficial 
crust,  the  first  rudimentary  state  of  that  solid 
ground  on  which  we  actually  live. 

This  crust  or  shell,  due  to  the  cooling  of  the 
exterior  layers  of  the  heated  and  rotating  spheroid, 
had  the  effect  of  preventing  the  rapid  cooling  of 
the  fused  layers  beneath,  which  thus  retained 
their  high  temperature.  Immediately  beneath 
the  solid  stratum  were  liquid  and  gaseous  masses 
in  motion,  while  near  the  Earth's  centre  the  fused 
matter,  liquid  or  even  gaseous,  subjected  to  the 
enormous  pressure  of  several  millions  of  atmos- 
pheres by  reason  of  the  weight  of  the  exterior 
layers,  probably  existed  in  a  condition  practically 
equivalent  to  the  solid  state,  compressed  as  it  was 
beyond  any  pressure  realisable  in  our  laboratories. 
The  heated  nucleus  would  contain  all  the  chemical 

24 


The  Age  of  the  EartH  25 

elements,  since  it  came  from  a  portion  detached 
from  the  solar  matter,  and  since  the  spectroscope 
proves  the  existence  of  all  these  elements  in  the  Sun. 
But  there  would  certainly  be  an  excess  of  iron,  for, 
in  the  first  place,  spectrum  analysis  of  the  light 
of  the  stars  teaches  us  that  iron  predominates  in 
them  from  the  first  phase  of  their  evolution. 
Secondly,  the  actual  general  magnetic  state  of 
the  Earth  at  the  present  time  indicates,  by  its 
effect  on  a  magnetised  needle,  the  presence  of 
magnetic  materials  in  considerable  quantity  at 
the  centre  of  the  globe.  Furthermore,  these 
materials  are  found  in  the  lava  which  flows  at 
intervals  from  volcanic  craters  over  the  surface 
of  the  Earth. 

De  Launay  has  shown  that  it  is  possible,  by 
means  of  geological  considerations,  to  assign  the 
order  of  superposition  of  the  most  widely  occurring 
chemical  elements,  at  the  time  when  the  Earth 
had  ceased  to  be  entirely  fluid.  The  elements 
may  in  this  way  be  classified  into  seven  groups, 
the  first  of  which  is  represented  by  hydrogen,  and 
the  last  of  which  contains  the  heavy  precious 
metals.  The  atomic  weights  would  increase  with 
the  depth  and  therefore  the  elements  would  be 
found  in  the  crust  at  distances  from  the  centre  in 
inverse  proportion  to  the  atomic  weights.  The 


26  The  EartH 

atoms,  freed  from  chemical  affinity  at  the  high 
temperature  in  question,  individually  obeyed,  in 
the  fluid  rotating  sphere,  only  the  laws  of  gravita- 
tion and  centrifugal  force. 

Above  the  crust  thus  formed  exists  an  atmos- 
phere which  is,  at  first,  at  the  temperature  of 
solidification  of  the  rock.  The  latter  was,  there- 
fore, formed  by  the  solidification  of  the  most 
refractory,  that  is  to  say  the  least  volatile,  elements. 
The  first  minerals  appearing  at  the  surface  would 
be  combinations  of  silica  with  alumina,  also  lime, 
magnesia,  and  a  little  iron  and  soda. 

The  terrestrial  crust,  which  was  at  first  very 
thin,  played  an  important  r61e;  it  separated  the 
interior  incandescent  nucleus  from  the  layer  of 
gases  and  vapours  which  surrounded  the  Earth. 
This  gaseous  envelope  or  atmosphere,  the  remain- 
ing constituents  of  which  envelop  our  globe  at  the 
present  time,  originally  contained  a  considerable 
proportion  of  carbon  dioxide  gas,  which  was 
emitted  continuously  by  the  turbulent  fluid  inte- 
rior matter.  It  contained  also  light  gases,  notably 
hydrogen  which  was  present  in  very  large  quan- 
tity; spectrum  analysis  demonstrates  its  existence 
in  the  atmospheres  of  the  distant  planets,  such  as 
Uranus  and  Neptune,  which  are  in  the  process  of 
evolution.  The  atmosphere  also  contained  hydro- 


THe  Age  of  the  EartH  27 

carbons  and  considerable  quantities  of  oxygen 
and  nitrogen. 

When  the  solid  terrestrial  crust  was  definitely 
formed,  it  was  at  a  very  high  temperature,  namely 
that  of  its  solidification.  It  could  not,  therefore, 
retain  light  gases  such  as  hydrogen  and  helium, 
which  were  dissipated  into  the  solar  nebula,  and 
thence  passed  out  into  intersidereal  space  where 
they  constituted  rudimentary  nebulae.  These 
gases  now  exist,  in  the  lower  regions  of  our  atmos- 
phere, only  in  very  minute  traces;  at  a  height  of 
100  kilometres  [62.5  miles]  from  the  ground  the 
little  atmosphere  which  remains  is  probably  com- 
posed approximately  of  99>£%  of  hydrogen  and 
}4%  of  helium. 

Thus,  when  the  crust  was  completely  formed, 
there  remained  as  atmospheric  constituents  a 
great  quantity  of  nitrogen  and  also  a  large  pro- 
portion of  carbon  dioxide  and  water  vapour,  for 
almost  all  the  oxygen  was  in  combination  with 
hydrogen,  forming  water,  which  the  high  tempera- 
ture prevented  from  condensing  to  liquid  form. 
Water  cannot  exist  in  the  liquid  state  above  the 
temperature  of  360°  C.  [680°  P.],  which  is  called 
its  critical  temperature. 

As  the  temperature  of  the  atmosphere  gradually 
fell,  the  most  volatile  metals  remaining  in  the 


28  The  Earth 

form  of  vapour,  such  as  potassium  and  sodium, 
were  the  first  to  condense.  Then,  as  the  cooling 
continued,  elements  which  the  high  temperature 
had  prevented  from  combining  were  now  able  to 
do  so,  and  thus  chlorides,  bromides,  iodides,  etc., 
were  produced.  When  the  temperature  had  de- 
creased to  below  the  critical  one  of  360°  C.  [680° 
F.],  the  water  vapour  began  to  be  precipitated 
in  the  liquid  state.  The  original  pressure  of  the 
atmosphere  must  have  been  very  considerable, 
since  it  contained  in  the  gaseous  condition  the 
whole  of  the  water  actually  existing  on  the  earth 
at  the  present  time.  Now,  if  the  oceans  were 
distributed  uniformly  over  the  Earth's  surface, 
they  would  form  a  layer  of  water  of  more  than 
3000  metres  [1.9  miles]  in  depth,  exercising  a 
pressure  of  300  times  that  of  our  present  atmos- 
phere; and  this  water  as  vapour  would  have 
exerted  an  equal  pressure  in  the  early  atmosphere. 
During  this  period,  the  solid,  but  still  thin, 
crust  was  continually  kept  in  a  state  of  agitation 
by  the  bubbling  up  of  the  internal  mass,  the  upper 
layers  of  which,  liquid  or  gaseous,  came  into  con- 
tact with  and  pressed  against  its  inner  surface. 
Under  these  repeated  attacks,  the  crust  gave  way 
in  places,  and  became  pierced  with  craters,  fissures, 
and  crevices  which  allowed  the  upward  pressing 


The  Age  of  the  EartH  29 

fused  matter  to  escape.  At  the  lower  temperature 
of  the  surface,  this  matter  solidified,  thus  giving 
rise  to  the  formation  which  geologists  call  Archaean, 
through  which  jets  of  the  interior  magma  burst 
forth,  producing  the  eruptive  rocks  on  solidifica- 
tion. This,  however,  is  not  all.  On  account  of 
the  continuous  cooling,  due  to  the  thinness  of  the 
primitive  crust,  the  latter,  not  being  completely 
sustained  by  the  interior  contracting  mass,  sank 
in  certain  parts  when  the  internal  pressure  raised 
up  other  parts.  The  outer  surface  of  the  solid 
part  of  the  globe,  that  is  the  surface  of  the  litho- 
sphere,  would  therefore  not  be  uniform ;  it  became 
wrinkled  and  indented,  presenting  protuberances 
and  hollows. 

When  the  atmosphere  cooled  to  the  critical 
temperature  of  360°  C.  [680°  P.],  and  the  water 
vapour  consequently  began  to  condense  to  the 
liquid  state,  the  latter  fell  as  scalding  rain  on  to 
the  solid  surface.  This  water  condensed  on  the 
higher  portions  of  the  surface  and  flowed  down  the 
declivities,  dissolving  a  greater  or  less  proportion 
of  all  the  substances  distributed  over  the  surface 
of  the  terrestrial  globe.  Thus,  the  streaming  of 
the  water  commenced  on  an  extensive  scale.  The 
water  accumulated  in  the  cavities,  the  folds,  and 
the  hollows  of  the  solidified  crust,  in  accordance 


30  The  EartK 

with  the  laws  of  gravity.  In  this  way  the  oceans 
first  came  into  being,  and  it  is  probable  that,  as 
they  resulted  from  the  accumulation  of  water 
which  was  originally  hot,  and  which  had  bathed 
the  entire  surface  of  the  Earth,  they  would  have 
dissolved  in  the  process  everything  that  could  be 
taken  into  solution  and  that,  therefore,  they 
would  contain,  at  any  rate  in  traces,  all  the  ele- 
ments which  were  to  be  found  in  the  enveloping 
crust  of  the  Earth. 

We  shall  now  examine  the  form  taken  by  this 
shell,  the  lithosphere,  the  hollows  of  which  re- 
ceived the  waters  of  the  primitive  seas,  and  whose 
higher  emerging  portions  constituted  the  original 
continents. 

If  the  superficial  crust  had  continued  to  envelop 
a  nucleus  which  sustained  it  at  every  point,  that 
is  to  say  with  which  it  was  in  perfect  contact,  this 
solid  stratum  would  have  simply  taken  the  form 
of  the  fluid  nucleus,  slightly  flattened  on  account 
of  the  centrifugal  force  due  to  the  Earth's  rotation. 
The  crust  would  thus  have  the  geometrical  form 
of  an  ellipsoid  of  revolution.  But  its  support  was 
imperfect,  on  account  of  the  slow  contraction  of 
the  central  mass  due  to  cooling,  and,  therefore, 
as  has  been  said  above,  it  became  folded  and 
wrinkled  and  covered  with  inequalities,  hollows 


The  Age  of  tKe  EartK  31 

in  some  parts,  protuberances  in  others.  These 
suffered  frequent  changes  in  the  early  periods,  but 
such  changes  became  rarer  and  less  widespread 
as,  in  the  course  of  time,  the  Earth  evolved  towards 
its  actual  present  condition. 

Were  these  foldings  produced  quite  by  chance 
as  might  be  supposed  from  a  superficial  examina- 
tion? We  know  that  chance  does  not  exist  and 
that  what  we  so  designate  is  only  the  resultant 
of  a  number  of  forces  or  conditions  of  which  we  are 
more  or  less  ignorant.  Everything  in  that  won- 
derful machine,  the  Universe,  is  regulated  by 
inflexible  laws.  The  foldings  of  the  terrestrial 
crust  were  not  produced  erratically;  their  forma- 
tion was  in  accordance  with  the  law  of  tetrahedral 
symmetry.  At  the  time  of  its  origin  the  crust  took 
a  certain  form  which  it  would  tend  to  preserve 
unchanged.  In  order  that  it  should  change  as 
little  as  possible,  when  the  interior  volume  came 
to  diminish  in  consequence  of  the  contraction  of 
the  nuclear  mass,  the  crust  should  have  a  regular 
figure  corresponding  to  the  minimum  content  for 
the  given  surface.  Geometry  teaches  us  that  the 
tetrahedron,  a  regular  solid  figure  with  four  tri- 
angular faces,  a  pyramid  with  a  triangular  base, 
satisfies  this  condition.  Many  causes  would 
operate  against  the  crust  taking  this  form  in  its 


THe  Earth 


entirety,  but  it  would  at  any  rate  indicate  by  the 
direction  of  its  folds  a  tendency  to  take  the  tetra- 
hedral  shape.  This  tendency  would  manifest  it- 
self in  the  diametrical  opposition  of  the  continents, 
since  these  represent  the  emergent  apices  of  the 
tetrahedron,  and  a  similar  opposition  in  the  case 
of  the  oceans  which  correspond  to  the  plane  faces 
of  the  pyramid.  These  faces  are  necessarily 

below  the  surface  of  the 
seas  which  thus  make  up 
the  flattened  spheroidal 
form  imposed  on  the 
Earth  by  the  combined 
action  of  the  laws  of 
gravity  and  centrifugal 
force.  (Fig.  4.)  Thus 
from  the  simple  fact  of 
the  contraction  of  the 
internal  mass  we  are  able  to  form  some  idea  of 
how  the  fundamental  division  of  the  surface  into 
land  and  water  came  about,  a  division  which  re- 
mained for  a  long  time  one  of  the  mysteries  of 
geographical  science. 

There  is  another  point  to  be  noted  about  this 
tendency  to  take  the  tetrahedral  form.  The 
edges,  or  aretes,  of  the  tetrahedron  have  also  an 
essential  significance;  they  indicate  the  general 


PIG.    4.— The  Terrestrial 
"Tetrahedron." 


The  Age  of  tHe  EartH  33 

orientation  of  the  emergent  land  which  runs 
roughly  north  and  south.  We  will  return  at  a 
fitting  time  to  this  important  subject  from  the 
point  of  view  of  the  figure  of  the  Earth. 

We  are,  henceforth,  able  to  distinguish  two 
very  distinct  things:  first,  the  lithosphere  which 
is  formed  by  the  solid  shell  of  the  globe,  a  shell 
originally  spheroidal  and  later  deformed  by  the 
foldings  and  furrows  of  its  surface  brought  about 
by  the  tetrahedral  laws,  at  any  rate  as  regards 
the  essential  features;  and,  second,  the  hydro- 
sphere, formed  by  the  water  surface,  the  fluidity 
of  which  causes  it  to  be  governed  by  the  laws  of 
gravity  and  of  rotation,  and  which  maintains, 
save  for  slight  local  perturbations  in  the  immediate 
neighbourhood  of  the  continental  masses,  the 
flattened  ellipsoidal  figure  which  is  a  necessary 
result  of  the  laws  of  mechanics. 

We  shall  have  to  return  in  fuller  detail  to  the 
tetrahedral  theory  in  the  course  of  this  work;  we 
shall  find  its  application  to  the  theory  of  seismic 
phenomena,  those  eruptions,  which  are  due  to  the 
impulse  of  the  heated  internal  mass  and  which 
constantly  agitate  and  dislocate  the  crust  of  the 
earth. 

The  condensation  of  the  atmospheric  water 
vapour,  which  began  at  a  high  temperature,  sub- 


34  The  Earth 

sequently  continued  and  its  extent  increased  as  the 
cooling  went  on.  The  temperature  fell  little  by 
little  and  when  it  reached  the  neighbourhood  of 
55°  C.  [131°  P.],  the  conditions  required  for  life 
were  realised.  Given  a  living  germ,  it  could  grow, 
reproduce  itself,  and  evolve,  that  is  to  say,  organ- 
ised beings  could  prosper.  Furthermore,  since 
the  cooling  was  not  rapid,  a  state  of  equilibrium 
was  established  between  the  total  heat  received 
from  the  Sun  and  from  the  heated  interior  of  the 
Earth  on  the  one  hand  and  the  loss  by  radiation 
on  the  other  hand,  in  such  a  way  that  the  condi- 
tions of  temperature  favourable  to  the  existence 
of  living  beings  were  brought  about  in  due  course. 
The  torrents  of  water  which  streamed  over  the 
continents  carried  the  debris  washed  therefrom 
into  the  sea  and  deposited  it  at  the  bottom  of  the 
oceanic  hollows.  Thus  the  process  of  sedimenta- 
tion commenced;  the  primitive  rocks  were  all 
of  igneous  origin,  but  now  other  kinds  began  to 
be  formed  by  the  superposition  of  successive 
deposits  on  these  original  rocks. 

Hence  was  instituted  the  history  of  the  early 
ages  of  the  Earth,  its  geological  history,  which  is 
that  of  the  time  prior  to  the  appearance  of  man 
on  the  globe. 

At  the  period  of  which  we  are  speaking,  the 


TKe  Age  of  the  Earth  35 

crust  was  a  rigid  shell  having  a  composition  ana- 
logous to  that  of  granite,  and  the  oceans  existed, 
but  frequent  changes  occurred  in  the  configuration 
of  the  continents  and  seas  in  consequence  of  the 
convulsions  of  the  still  weak  crust,  under  the 
influence  of  the  outward  thrust  of  the  interior 
mass.  The  primitive  atmosphere  was  rich  in 
water  vapour  and  carbon  dioxide  and  did  not  yet 
contain  all  the  oxygen  necessary  to  maintain  life, 
a  part  of  which  still  remained  combined  with 
carbon.  Thick  clouds  floated  in  it  on  account 
of  the  superabundance  of  uncondensed  water 
vapour. 

The  large  proportion  of  carbon  dioxide  in  the 
atmosphere  at  this  period  gave  to  the  latter  a 
remarkable  property  which  the  actual  atmosphere 
at  the  present  time  does  not  possess  to  anything 
like  the  same  extent.  It  played  the  part  of  a 
protective  screen,  keeping  in  the  heat  and  conse- 
quently lessening  the  rate  of  the  Earth's  cooling. 
Carbon  dioxide  now  constitutes  scarcely  ^Vo 
part  of  the  air,  but  calculations  based  on  ex- 
perimental evidence  have  led  Arrhenius  to  the 
conclusion  that  if  this  small  quantity  of  gas  were 
absent  the  temperature  of  the  Earth's  surface 
would  fall  21°  C.  [37.8°  F.].  This  would  further 
lead  to  the  condensation  of  a  large  part  of  the 


36  THe  Earth 

aqueous  vapour  still  present.  As  this  also  acts 
as  a  retaining  screen  in  the  same  way  as  the  carbon 
dioxide  does,  it  will  be  seen  that  the  disappearance 
of  this  gas  would  bring  disaster  upon  the  Earth, 
from  the  point  of  view  of  temperature. 

• 

Conversely,  it  will  be  readily  understood  how 
in  the  early  ages  of  the  Earth's  history  the  protect- 
ing mantle,  formed  by  an  atmosphere  considerably 
richer  in  carbon  dioxide  and  water  vapour  than 
our  present  one,  enabled  the  soil  to  maintain  the 
high  temperature  that  caused  the  extraordinary 
development  of  vegetation  characterising  that 
period. 

From  the  epoch  of  the  solidification  of  the  crust 
up  to  the  present  time,  the  history  of  the  Earth  is 
called  Geology.  It  is  outside  the  scope  of  the 
present  work  to  trace  it  in  all  its  details.  M.  de 
Launay  has  given  an  authoritative  exposition  of 
it  in  his  masterly  work  The  History  of  the  Earth. 
We  will  confine  ourselves  here  to  an  outline  of  the 
chief  facts. 

That  part  of  the  original  solid  crust  which  was 
covered  by  the  oceans  due  to  the  condensation  of 
the  atmospheric  water  vapour,  oceans  that  were 
destitute  of  beaches,  constituted  the  foundation 
upon  which  all  the  later  solid  formations  came  to 
be  built  up.  The  first  disturbances  of  the  primi- 


The  Age  of  tKe  Earth  37 

tive  crust,  the  first  foldings  that  it  experienced, 
produced  high  lands  and  depressions,  thus  fixing 
the  original  distribution  of  the  continents  and 
seas.  The  Archaean  rocks,  that  are  invariably 
met  with  when  the  soil  is  penetrated  deeply  enough 
to  get  below  all  the  superincumbent  strata,  are 
the  oldest  known  rocks.  During  the  period  of 
formation  of  the  Archaean  rocks  eruptions  from 
the  central  mass  into  crevices  of  the  thin  crust 
were  frequent  and  the  Plutonic  rocks  were  pro- 
duced by  the  solidification  of  the  interior  material 
thus  pushed  up.  In  fact  by  the  study  of  the 
Earth's  crust  we  find  only  the,  granitic  or  Plutonic 
rocks  underlying  the  crystalline  or  Archaean  rocks. 

The  great  thickness  of  the  Archaean  formation, 
which  in  certain  regions  is  10,000  metres  [6J 
miles]  or  even  more,  indicates  the  enormous  dura- 
tion of  this  first  period  of  the  Earth's  history. 

The  rocks  formed  were  still  at  a  high  tempera- 
ture and  the  primitive  atmospheric  condensation 
brought  down  scalding  liquids,  so  that  the  con- 
ditions at  this  time  were  not  suited  to  animal 
or  vegetable  life.  It  is,  therefore,  not  remarkable 
that  we  find  no  trace  of  any  living  thing  in  these 
first  strata.  Possibly  elementary  life  made  its 
appearance  at  the  end  of  this  period,  but  any  such 
creatures  being  destitute  of  hard  or  bony  struc- 


38  The  Earth 

tures  left  no  trace  of  their  existence  on  rocks  so 
hard  and  at  the  same  time  so  convulsed  as  those 
which  form  the  Archaean  strata. 

In  proportion  as  the  atmospheric  temperature 
and  therefore  that  of  the  first  oceans  fell  and 
reached  the  neighbourhood  of  60°  C.  [140°  P.], 
the  terrestrial  conditions  began  to  be  such  as 
would  admit  the  possibility  of  life.  But  how  did 
life  make  its  first  appearance  in  the  world?  Per- 
haps wandering  cells  driven  from  another  world 
by  the  pressure  of  radiation  reached  the  Earth 
and  lived  and  evolved  thereon,  having  resisted 
the  influence  of  cold  during  their  long  journey 
through  space,  the  possibility  of  which  resistance 
has  been  demonstrated  by  work  in  the  laboratory 
at  Leyden.  Arrhenius  is  of  opinion  that  they 
must  also  have  escaped  from  the  destructive  ac- 
tion of  the  ultra-violet  rays.  Or  perhaps  life 
originated  on  our  globe  in  some  other  unknown 
way.  The  problem  is  one  which  is  at  present 
unsolved  and  will  doubtless  remain  so  for  a  long 
time.  What  is  certain  is  that  the  Primary  Era, 
characterised  by  the  appearance  of  life,  vestiges 
of  which  remain  to  us  as  animal  and  vegetable 
fossils  in  the  strata  of  this  period,  began  after  the 
stage  represented  by  the  Archaean  rocks.  The 
strata  corresponding  to  this  era  are  classified 


TKe  Age  of  the  Earth  39 

by  geologists  into  Silurian,  Devonian,  Carboni- 
ferous, and  Permian. 

As  previously  explained,  the  atmosphere,  rich 
in  carbon  dioxide  and  water  vapour,  formed  around 
the  earth  a  protective  screen  preventing  rapid 
cooling  and  maintaining  an  extremely  high  tem- 
perature at  the  surface  of  the  ground.  Conse- 
quently vegetable  life  all  through  the  Primary 
Era,  but  particularly  in  the  Carboniferous  period, 
flourished  with  an  extraordinary  fertility.  The 
remains  which  are  found  in  coal-beds  show  that 
vegetable  species,  which  are  to-day  merely  small 
plants,  were  then  veritable  trees,  forming  great 
forests.  There  were  at  first  only  cryptogams  and 
later  also  gymnosperms.  With  regard  to  animals 
it  can  be  affirmed  that  life  commenced  in  the 
seas.  The  first  beings  were  invertebrates;  it  is 
only  at  the  end  of  the  Primary  Era  that  the  first 
fishes,  having  vertebrate  bony  systems,  are  found. 
There  were  neither  birds  nor  mammals.  It  is 
however  remarkable  that  the  first  animals  whose 
remains  can  be  found,  the  trilobites,  show  an 
organisation  sufficiently  high  to  indicate  that 
they  were  products  of  an  already  advanced  evolu- 
tion. It  is  a  far  cry  from  elementary  cells  to 

trilobites, 
~^^r^t  _"  i-'-trt-S*" 

The  great  activity  of  the  primary  vegetation 


40  The  EartK 

has  had  a  decisive  influence  on  the  history  of  the 
globe.  The  absorption  of  carbon  dioxide  by 
the  abundant  vegetation  restored  free  oxygen  to 
the  terrestrial  atmosphere  and  produced  little  by 
little  the  quantity  actually  present.  Furthermore, 
the  minerals  first  formed,  combinations  of  silica 
with  lime,  alumina,  magnesia,  iron,  and  soda,  had 
gradually  been  attacked  by  the  carbon  dioxide  of 
the  primitive  atmosphere,  largely  by  the  agency 
of  water  which  contained  the  gas  in  solution  on 
account  of  its  prevalence  in  the  air.  Lime,  magne- 
sia, soda,  and  iron  were  thus  converted  into  soluble 
carbonates  and  accordingly  carried  down  to  the 
seas  by  the  water-streams  and  accumulated  there. 
The  first  living  beings  assimilated  these  substances, 
as  their  remains,  deposited  in  sedimentary  layers, 
testify.  In  fact  the  formation  of  sedimentary 
limestone  and  dolomite  required  34,000  times 
more  carbon  dioxide  than  is  actually  present  in 
the  air.  Thus  large  quantities  of  this  gas  must 
have  been  removed  from  the  atmosphere  in  ad- 
dition to  what  was  decomposed  by  vegetable  life. 

It  can  legitimately  be  stated  that  almost  the 
whole  of  the  actual  free  oxygen  of  the  air  is  due  to 
the  vegetation,  especially  that  of  the  Primary  Era. 

An  era  of  calm  and  stability  then  succeeded. 
This  is  called  the  Secondary  Era  and  geologists  sub- 


The  Ag'e  of  the  Earth  41 

divide  it  into  the  Triassic,  Jurassic,  and  Cretaceous 
periods.  It  commenced  as  the  atmosphere,  by 
the  gradual  diminution  of  carbon  dioxide  and  the 
increase  of  oxygen,  became  more  and  more  suit- 
able to  the  development  and  evolution  of  living 
creatures.  Other  characteristics  were  the  gradual 
decrease  of  temperature,  which  was  still  high,  and 
the  greater  stability  of  the  Earth's  crust,  strength- 
ened and  thickened  by  successive  solidifications. 
The  fossil  remains  of  life  which  are  met  with  in 
these  strata,  which  are  always  superimposed  on 
Archaean  or  Primary  formations,  clearly  differen- 
tiate this  era  from  the  preceding  one.  As  regards 
the  vegetation,  the  prevalence  of  cryptogams 
ceased  in  the  Secondary  Era  while  gymnosperms 
were  everywhere  predominant.  The  first  living 
beings  whose  traces  have  been  found,  the  trilobites 
of  the  Primary  Era,  had  completely  disappeared. 
In  their  stead  were  belemnites,  the  forerunners 
of  our  present  cuttlefish,  and  ammonites  which 
were  cephalopods  with  spiral  shells;  these  were 
characteristic  of  the  era.  In  the  seas,  crinoids, 
sponges,  and  corals  were  abundant  and  forami- 
nifera  and  radiolaria  developed.  Their  hard  parts 
accumulated  in  thick  layers  and  by  this  sedimen- 
tation process  covered  the  bottoms  of  the  seas  of 
that  period.  Thus  the  work  of  the  ocean  began, 


42  THe  EartH 

a  work  which  still  continues  without  cessation  in 
our  present  seas;  geology  is  mainly  the  oceano- 
graphy of  the  past,  and  the  study  of  oceanography 
forecasts  the  geology  of  the  future. 

But  the  characteristic  feature  of  the  Secondary 
fauna  was  the  evolution  of  vertebrate  animals; 
the  armoured  fish  of  the  Primary  period  had  dis- 
appeared, having  little  by  little  given  place  to  fish 
with  well-ossified  vertebrae.  In  particular,  gigantic 
reptiles  came  into  being,  the  colossal  size  of  whose 
skeletons  fills  us  with  astonishment.  The  ich- 
thyosaurus, the  plesiosaurus,  and  the  mosasaurus 
were  the  monsters  that  peopled  the  seas,  while  the 
continents  were  inhabited  by  immense  creatures, 
dinosaurs,  some  of  which  attained  a  length  of 
twenty-five  metres  [82  ft.]  and  a  considerable 
height.1  When  one  sees,  in  the  palaeontological 
galleries  of  museums,  the  skeletons  of  iguanodon, 
brontosaurus,  stegosaurus,  and  diplodocus,  one 
cannot  help  being  struck  by  the  manifestation  of 
strength  that  these  gigantic  animals  represent. 
When  standing  erect,  some  of  them  would  have 
overtopped  the  roof  of  an  ordinary  five-storey 
house.  Finally,  the  first  birds  made  their  appear- 
ance in  Secondary  times,  and  when  the  gigantic 

1  Quite  recently,  remains  of  a  huge  creature  about  double 
this  length  have  been  found. — Trans. 


THe  Age  of  the  Earth  43 

cold-blooded  animals,  which  have  just  been  men- 
tioned, were  predominant,  in  consequence  of  their 
size  and  strength,  much  smaller  animals,  the  first 
warm-blooded  mammals,  had  their  origin.  The 
geography  of  these  times  was  characterised  by  two 
continents  in  the  northern  hemisphere  separated 
by  an  ocean  in  the  midst  of  which  was  emergent 
land  corresponding  to  the  actual  situation  of 
northern  Europe.  In  the  southern  hemisphere 
a  vast  continent  stretched  over  the  position  of 
South  America  and  Africa;  the  South  Atlantic 
Ocean  did  not  exist  and  land  marked  the  future 
place  of  Australia.  Volcanic  eruptions  were  less 
frequent,  the  Secondary  Era  being,  as  above 
remarked,  a  period  of  relative  tranquillity. 

This  state  of  calm  came  however  to  an  end  and 
gave  place  to  violent  convulsions;  the  volcanoes 
manifested  great  activity,  increasing  to  a  state  of 
paroxysm,  and  at  the  same  time  arose  the  great 
mountain  chains  which  are  actually  present  on  the 
globe.  During  this  time  animal  life  was  gradually 
perfecting  its  forms  and  was  producing  creatures 
more  and  more  resembling  present  ones;  and  mam- 
mals, some  of  which  attained  gigantic  dimensions, 
were  masters  of  the  land  surfaces.  As  examples, 
we  have  the  palaeotherium,  the  hipparion,  the 
colossal  dinotherium,  the  mastodon  (the  first 


44  The  Earth 

elephant),  the  hippopotamus,  the  rhinoceros,  the 
great  deer,  ruminant  and  carnivorous  animals. 
This  formed  the  Tertiary  Era,  subdivided  by  geo- 
logists into  the  Eocene,  Oligocene,  Miocene,  and 
Pliocene  periods.  Palms  abounded  at  first,  but 
towards  the  end  of  the  era  trees  appeared  which 
resembled  those  of  our  present  forests,  while  trop- 
ical flowers  narrowed  their  habitat  to  the  neigh- 
bourhood of  the  equator. 

The  emergent  lands  approached  more  and  more 
to  the  present  continental  contours.  The  great 
tertiary  chains  of  the  alpine  type  which  now  sur- 
round the  Mediterranean  basin  arose  as  a  result 
of  mountain-forming  movements  of  great  inten- 
sity. After  the  retreat  of  the  sea  which  invaded 
certain  parts  of  France,  the  shocks  recurred  and 
volcanic  eruptions  became  formidable ;  the  Central 
Plateau  became  covered  with  craters  which  emit- 
ted the  lavas  now  visible  in  Auvergne,  and  the 
actual  features  of  the  land  surface  gradually 
became  established. 

During  this  time,  the  atmosphere  continued  to 
lose  carbon  dioxide  and  water  vapour,  and  the 
cooling  of  the  Earth  by  radiation  became  greater, 
so  that  the  temperature  fell  little  by  little,  still 
remaining,  however,  higher  than  the  mean  tem- 
peratures now  observed  in  the  same  regions.  The 


The  Age  of  the  Earth  45 

nature  of  the  remains  of  vegetation  shows  that 
the  mean  temperature  of  France  was  more  than 
25°  C.  [77°  P.],  that  is  to  say  the  climate  of  that 
country  was  similar  to  that  which  characterises 
the  equatorial  regions  at  the  present  time.  It 
was  only  at  the  end  of  the  Tertiary  Era  that  the 
temperature  was  lowered  and  glaciers  appeared 
on  the  highest  mountains  and  commenced  their 
extension  towards  the  lower  regions.  When  this 
occurred,  the  fauna  and  flora  of  the  warm  climate 
gradually  receded  towards  the  tropics,  abandoning 
the  northern  lands  where  the  initial  climatic 
conditions  of  the  Tertiary  Era  had  allowed  them 
to  flourish,  but  whose  more  rigorous  later  climate 
was  too  cold. 

The  Earth  slowly  attained  its  present  aspect. 
The  vegetation  had  developed  into  the  forms  with 
which  we  are  now  familiar ;  the  animals  had  evolved 
and  had  reached  a  kind  of  perfection.  The  en- 
vironment was  thus  ready  for  the  existence  and  de- 
velopment of  the  creature  which  came  to  dominate 
nature,  that  is  to  say  Man,  and  the  Quaternary 
Era  began. 

The  Quaternary  strata  have  a  very  different 
character  from  the  preceding  ones;  exterior  agen- 
cies predominated  in  forming  them.  They  cover 
all  the  others  and  are  themselves  covered  only 


46  THe  Earth 

by  the  soil-cap.  They  are  alluvial  deposits,  the 
consequence  of  enormous  precipitations  of  rain, 
due  to  the  condensation  of  water  vapour  on  a 
large  scale,  following  the  great  lowering  of  tem- 
perature caused  by  the  almost  complete  ab- 
sorption of  carbon  dioxide.  These  abundant 
precipitations,  which  probably  constitute  the  origin 
of  the  story  of  the  Deluge  which  exists  among  all 
peoples,  led  to  great  rivers.  Snowfalls,  prevented 
from  melting  by  the  fall  of  temperature,  caused 
an  enormous  extension  of  the  glaciers,  which  at 
that  time  covered  the  whole  of  Central  Europe 
and  all  North  America.  The  ground  is  covered 
with  erratic  blocks,  indisputable  evidence  of  the 
existence  of  glaciers  in  the  first  part  of  the  Quater- 
nary Era,  to  which  geologists  have  given  the  name 
of  Pleistocene  epoch.  This  glaciation  consequently 
led  to  the  migrations  of  animals,  because  of  the 
great  climatic  variations  which  resulted  from  it. 

These  rivers  deposited  the  Quaternary  strata, 
in  which  may  be  found  precious  stones,  gold,  and 
platinum.  Above  the  Pleistocene  deposits  are 
the  different  recent  strata  composed  of  clay,  fine 
sand,  and  silt  which  are  utilised  for  cultivation. 

At  the  end  of  the  Quaternary  Era,  the  volcanoes 
of  Auvergne  again  became  active,  and  distinct 
evidence  of  this  relatively  recent  renewed  activity 


The  Ag'e  of  tHe  EartK  47 

may  be  seen  at  the  present  time  on  the  Puys  chain. 
Later  on,  the  glaciers  retreated,  and  the  present 
climatic  conditions  established  themselves  by 
degrees. 

The  great  mammals,  the  mammoth,  rhinoceros, 
cave  bear,  and  great  elk  have  since  disappeared; 
so  also  has  the  megatherium  of  South  America. 
Diminutive  specimens  of  the  wingless  birds  from 
which  ostriches  and  cassowaries  are  derived  still 
exist  in  New  Zealand  and  are  called  kiwis,  but 
they  are  rare. 

It  is,  also,  in  this  latter  country  that  the  least 
civilised  natives  are  found,  natives  who  approach 
the  nearest  to  a  purely  natural  condition  and  who 
are  able  to  afford  us  some  idea  of  primitive  man. 

Man  made  his  appearance  on  the  Earth  in  the 
Quaternary  Era.  His  presence  is  proved  by  the 
remains  of  human  bones,  which  have  as  yet  only 
been  found  in  Quaternary  strata,  never  in  any  of 
the  preceding  formations,  and  also  by  the  remains 
of  objects  unmistakably  his  handiwork.  In  the 
earliest  stage,  during  the  prehistoric  period,  are 
found  only  implements  formed  of  hard  stone; 
flints  rudely  hewn  by  chipping.  This  is  known  as 
the  Palaeolithic  period,  that  of  chipped  stone, 
which  gave  place  to  that  of  polished  stone,  the 
Neolithic  period.  Subsequently  to  the  latter, 


48  The  Earth 

metals  began  to  be  worked,  first  of  all  bronze,  in 
the  bronze  age,  and  secondly  iron,  in  the  iron  age. 

The  history  of  mankind  commenced  from  this 
time. 

Such  are  the  stages  passed  through  by  the  Earth 
in  the  course  of  its  early  years,  during  its  infancy 
and  youth,  before  arriving  at  the  state  of  maturity 
to  which  it  has  now  attained.  One  very  import- 
ant question  suggests  itself  to  the  mind :  How  many 
years  has  that  slow  evolution  taken?  Or  in  other 
words,  what  actually  is  the  age  of  the  Earth? 
It  is  very  difficult  to  give  an  answer,  at  any  rate 
a  precise  one,  but  in  default  of  an  exact  knowledge 
of  the  length  of  time  the  Earth  has  been  in  exist- 
ence, we  may  get  some  idea  of  the  magnitude  of 
the  period  which  has  elapsed  since  the  crust  solidi- 
fied and  enclosed  the  heated  nucleus,  the  result  of 
its  stellar  origin. 

This  evaluation  may  be  approached  in  different 
ways.  We  may  for  example  ask  ourselves  what 
time  must  have  elapsed  in  order  for  the  oceans  to 
have  acquired  their  actual  salinity,  by  the  accu- 
mulation of  material  that  the  streams  brought 
down  in  solution  from  the  solid  crust  over  which 
they  flowed.  Joly  has  attempted  this  estimation. 
He  calculated  how  much  salt  the  rivers  annually 
carry  to  the  ocean  and  by  comparison  of  this 


The  Age  of  the  EartH  49 

quantity  with  the  amount  that  sea- water  actually 
contains  he  arrived  at  the  conclusion  that  at  least 
a  hundred  million  years  must  have  been  necessary 
for  the  present  salinity  to  have  been  acquired  in 
this  way.  It  will  not  serve  a  useful  purpose  to 
describe  how  this  estimation  was  attained.  At 
the  beginning  of  the  aqueous  condensation,  the 
water  flowed  at  a  high  temperature  over  the  lands 
then  formed  and  so  it  dissolved  much  more  of 
saline  substances  than  can  the  cold  water  of  the 
rivers  which  at  the  present  time  flow  into  the  sea. 
For  this  reason,  however  ingenious  the  above 
estimate  may  be,  it  furnishes  us  with  very 
uncertain  data  as  to  the  Earth's  age. 

The  phenomenon  of  sedimentation  enables  us 
to  arrive  at  a  much  more  probable  evaluation, 
which  Sir  Archibald  Geikie  has  made.  If  the 
total  thickness  of  the  sedimentary  deposits  form- 
ing the  stratification  of  our  globe  be  estimated  at 
about  30,000  metres  [19  miles],  and  if  it  be  assumed, 
as  the  work  of  geologists  has  shown  that  between 
three  thousand  and  twenty  thousand  years  are 
required  for  a  layer  one  metre  [39.37  in.]  thick  to 
be  laid  down,  it  follows  that,  in  round  numbers, 
the  time  necessary  for  the  depositing  of  all  the 
known  strata  is  between  a  hundred  and  a  thou- 
sand millions  of  years.  This,  moreover,  takes  no 


50  THe  Earth 

account  of  Pre-Cambrian  formations  which  have 
existed  perhaps  as  long  again. 

The  discovery  of  the  phenomena  of  radioactivity 
made  by  the  French  physicist  Henri  Becquerel,  and 
the  important  researches,  which  these  discoveries 
have  led  to,  have  given  modern  geo-physicists 
another  basis  of  estimation.  It  is  known  that 
the  emanation  of  radioactive  substances,  such 
as  radium,  thorium,  or  even  uranium,  becomes 
transformed  into  helium.  The  English  physicist 
Rutherford,  to  whom  we  owe  the  discovery  of  the 
emanation,  has  determined  by  experiment  how 
much  a  given  weight  of  uranium  or  thorium  loses 
as  helium  in  the  course  of  a  year.  Also,  Sir  Wil- 
liam Ramsay  has  studied  the  minerals  from  which 
uranium  and  thorium  can  be  extracted  and  has 
determined  the  proportion  of  helium  therein 
contained.  From  his  results,  Rutherford  states 
that  at  least  four  hundred  million  years  must  have 
been  required  for  these  minerals  to  be  formed  in 
their  present  state.  It  will  be  seen  that  this 
result  is  in  harmony  with  the  result  deduced  from 
sedimentation,  being  at  any  rate  of  the  same  order 
of  magnitude. 

The  phenomena  of  radioactivity  which  have 
also  been  brought  into  requisition  to  explain  the 
constancy  of  the  emission  of  heat  from  the  Sun, 


The  Age  of  tKe  Earth  51 

have  enabled  us,  in  recent  years,  to  estimate  the 
age  of  the  Earth  with  more  and  more  precision. 
English  physicists,  in  particular,  have  done  not- 
able work  in  this  direction.  Starting  from  the 
quantity  of  helium  contained  in  minerals  the  fol- 
lowing duration  periods  have  been  assigned :  three 
million  years  to  the  Greensand,  six  million  to  the 
basaltic  rocks  of  Auvergne,  fifty-four  million  to  cer- 
tain Norwegian  rocks,  two  hundred  and  eighty-six 
million  to  some  of  the  rocks  of  Ceylon,  three  hun- 
dred and  twenty  million  to  the  blue  earth  of  Kim- 
berley,  and  six  hundred  million  to  the  Archaean 
formation  of  Ontario.  Thus  figures  similar  to  those 
of  Joly,  Geikie,  and  Rutherford  are  reached.  A 
study  of  the  Swedish  rock-masses  leads  to  still 
greater  figures,  the  age  indicated  for  them  being  a 
thousand  or  thirteen  hundred  million  years.  Some 
American  formations  give  results  of  thirteen  and 
fourteen  hundred  million,  and,  in  conclusion, 
specimens  of  rock  from  the  neighbourhood  of 
Colombo  in  Ceylon  have  had  assigned  to  them 
an  age  of  more  than  sixteen  hundred  million  years. 
Thus,  the  maximum  result  of  estimations  based 
on  the  duration  of  sedimentation,  viz.,  one  thou- 
sand million  years,  is  surpassed.  We  shall  find 
that  the  figures  in  question  are  confirmed  by  quite 
different  considerations,  of  an  essentially  geo- 


52  The  Earth 

graphical  character.  For  geographers  have  also 
contributed  knowledge  of  the  Earth's  age.  They 
have  studied  the  folds  of  which  we  have  spoken 
previously  and  which  constitute  our  mountain 
chains.  These  foldings  were  caused  by  the  fact 
that,  owing  to  the  cooling  and  contraction  of  the 
nucleus  on  which  it  originally  rested,  the  crust  was 
no  longer  sustained  from  below  and  consequently 
contracted,  becoming  shrivelled  as  in  the  case  of 
the  skin  of  a  fruit  when  it  dries  and  becomes 
smaller.  If  the  surface  area  of  the  mountain 
chains  be  measured  in  square  kilometres,  not  in 
projection,  as  upon  maps,  but  in  reality  on  their 
sides,  it  is  found  that  this  total  area  is  about  one 
hundred  and  fiftieth  of  the  entire  surface  of  the 
globe.  The  corresponding  decrease  in  the  length 
of  the  Earth's  radius  can  be  deduced;  it  is  a  little 
less  than  rJ-o  part  of  its  value,  and  this  contrac- 
tion would  correspond  to  a  lowering  of  tempera- 
ture of  more  than  300°  C.  [572°  F.].  To  produce 
this  fall  nearly  two  thousand  millions  of  years 
must  have  elapsed. 

As  a  result  of  all  that  has  been  said,  we  may 
consider  it  probable  that  the  actual  age  of  the 
Earth  lies  between  one  thousand  and  two  thou- 
sand million  years.  It  is  both  interesting  and 
very  remarkable  that  estimations  based  on  such 


The  Ag'e  of  the  EartK  53 

different  methods  give  results  that  are  sensibly 
concordant. 

Finally,  before  concluding  this  account  of  the 
Earth's  history,  we  have  to  ask  ourselves  whether 
the  terrestrial  crust  on  solidification  was  of  a 
uniform  thickness  surrounding  the  fluid  nucleus, 
or  whether  the  foldings  produced  during  the  first 
movements  of  the  shell  had  an  influence  on  the 
thickness  of  the  solid  stratum. 

Evidently  the  crust  did  not  solidify  as  a  whole 
at  one  time.  It  would  pass  through  stages  similar 
to  those  that  can  be  observed  in  baths  of  molten 
metal.  When  solidification  begins,  solid  crusts 
or  scoriae  form  in  places  and  float  on  the  surface 
of  the  rest  of  the  liquid  mass.  Certain  astrono- 
mers have  put  forward  the  rather  daring  theory 
that  the  spots  on  the  Sun  are  simply  the  first 
scoriae  so  formed,  indicating  the  beginning  of  a 
partial  solidification.  On  this  theory,  matter 
thrown  to  a  distance  by  the  solar  eruptions  con- 
sequently cools  and  falls  back  on  to  the  liquid  sur- 
face, on  which  it  floats  as  icebergs  float  on  water. 

It  is  very  probable  that  this  occurred  in  the 
case  of  the  Earth  during  the  formation  of  its 
crust;  solid  pieces  or  plates,  separate  from  each 
other  and  floating  in  the  main  fluid  mass,  were  first 
formed,  Lippmann  has  suggested  the  following 


54 


The  EartK 


ingenious  hypothesis.  Since,  as  he  says,  the 
crust  resembles  a  kind  of  irregular  mosaic  formed 
of  floating  fragments  in  juxtaposition  to  each 


FIG.  5A. — Lippmann's  Hypothesis  on  the  Earth's  Crust. 

other,  each  portion  must  be  sustained  from  below 
by  a  sufficient  upward  thrust  exerted  by  the  fluid 
mass  in  which  it  floats.  If,  therefore,  a  given 
piece  carries  a  considerable  mountain  mass  the 

weight  of  the  load  is 
much  greater  than  in 
the  case  of  a  piece 
above  which  there  is 
a  sea,  particularly  as 
the  density  of  the 
solid  mass  is  so  much 
higher.  It,  therefore, 
Wr  T .  follows  that  the  float - 

FIG.  SB. — Lippmann's  Hypothesis. 

ing  portion  of  greater 

weight  sinks  to  a  deeper  level  in  the  incandescent 
fluid  than  the  other.  It  has  a  greater  draught, 
to  use  a  nautical  expression  (Fig.  5A),  and  con- 
sequently the  crust  should  be  thicker  under  the 
continents  than  under  the  oceans  (Fig. 


The  Age  of  the  Earth  55 

We  shall  see  later  on  how  this  hypothesis  accords 
with  recent  determinations  of  the  intensity  of 
gravitation. 

We  have,  if  not  a  precise  value,  at  any  rate  some 
idea  of  the  order  of  magnitude  of  the  age  of  the 
Earth.  Geologists  estimate  the  duration  of  the 
respective  eras  as  follows:  75  per  cent,  for 
the  Primary  Era,  19  per  cent,  for  the  Secondary 
Era,  and  6  per  cent,  for  the  Tertiary  Era. 


CHAPTER  III 

THE  FORM,  MAGNITUDE,  AND  MASS  OF  THE  EARTH 

"\  X  TE  have  now  reviewed  the  successive  states 
*  *  through  which  the  Earth  has  passed  before 
reaching  that  with  which  we  are  familiar  to-day. 
The  periods  with  which  we  have  dealt  in  the 
course  of  the  preceding  two  chapters  correspond  to 
the  birth,  the  infancy,  the  adolescence,  and  the 
youth  of  the  Earth.  To-day  it  is  mature;  let  us 
see  in  what  manner  it  "exists"  and  how  it  "lives." 

We  must  first  of  all  gain  some  idea  of  its  external 
aspect,  and  we  will  commence  by  investigating 
its  form  and  dimensions. 

The  proofs  that  the  Earth  is  a  spheroidal  body, 
isolated  in  space,  have  been  summarised  in  our 
earlier  pages.  It  is  possible  to  give  a  preliminary 
notion  of  its  dimensions  by  remarking  that  a 
telescope,  whose  axis  is  truly  horizontal,  mounted 
on  the  summit  of  a  mountain  overlooking  the  sea, 
would  not  show  the  sea  horizon.  In  order  to  have 
this  horizon  in  view  in  the  centre  of  the  field  of 

56 


Form  and  Mass  of  tHe  EartH          57 

the  instrument,  the  latter  must  be  rotated  down- 
wards through  an  angle  which  astronomers  call 
the  " angle  of  depression."  If  this  angle  be  care- 
fully measured,  and  if  the  height  of  the  mountain 
be  also  known,  we  can  deduce  the  Earth's  radius 
by  means  of  elementary  geometry,  on  the  assump- 
tion that  it  is  a  true  sphere.  As  a  first  approxima- 
tion, the  result  so  obtained  is  6,366,000  metres 
[4000  miles].  It  is  noteworthy  that  if  this  experi- 
ment be  repeated  in  different  parts  of  the  Earth 
nearly  the  same  result  is  always  obtained.  We 
may  therefore  assert  that  the  Earth  is  sensibly 
spherical  and  that  its  radius  is  6,366,000  metres, 
to  a  first  approximation. 

If  we  now  make  a  more  precise  and  accurate 
determination  of  the  angle  of  depression  by  employ- 
ing a  more  powerful  telescope,  capable  of  rotation 
about  a  more  exactly  divided  circle,  and  if,  further, 
we  increase  the  magnitude  of  the  angle  to  be 
measured  by  taking  our  station  on  a  high  mountain 
surrounded  by  sea,  for  example  the  summit  of 
the  Peak  of  Teneriffe,  an  unexpected  result  is  ob- 
tained. Measured  in  the  north  direction  the  angle 
of  depression  is  greater  than  when  the  instrument 
is  pointing  east  or  west.  In  the  case  of  the  Peak 
of  Teneriffe,  which  rises  for  3710  metres  [12,000 
ft.]  above  sea-level,  the  difference  between  the  two 


58  TKe  Earth 

values  so  measured  is  twenty-eight  seconds  of  arc. 
Hence  it  may  be  deduced  that  the  Earth  must 
have  the  form,  not  of  a  perfect  sphere,  but  of 
an  ellipsoid  of  revolution,  flattened  at  the  poles 
and  bulging  at  the  equator.  The  polar  flattening 
may  even  be  determined  from  this  difference,  and 
is  found  to  be  TO-Q  part  of  the  equatorial  radius. 
This  flattening  is  a  necessary  consequence  of  the 
original  formation  of  the  Earth ;  while  still  fluid  it 
rotated  around  the  line  joining  the  poles,  and  the 
centrifugal  force  resulting  from  the  rotation  pro- 
duced the  polar  flattening  and  the  equatorial 
bulge. 

The  above-mentioned  method  of  measuring 
the  dimensions  of  the  Earth  is  subject  to  error, 
because  of  the  deviation  produced  in  luminous 
rays  slightly  inclined  to  the  horizon  by  atmospheric 
refraction.  It  is,  therefore,  necessary  to  find  a 
more  accurate  way  of  arriving  at  the  required 
result.  Nevertheless  it  gives  us  some  knowledge 
of  the  general  shape  of  our  globe  and  furnishes  us 
with  sufficiently  accurate  data  to  form  an  idea  of 
the  order  of  its  dimensions. 

Maps  of  the  Earth  show  that  water,  in  the  form 
of  oceans  and  seas,  covers  nearly  three-fourths 
of  its  surface.  Oceanographers,  from  the  time  of 
Maury  to  that  of  the  Prince  of  Monaco,  have 


Form  and  Mass  of  tHe  EartH         59 

explored  the  sea-depths  by  soundings,  and  the 
greatest  depth  reached  is  not  quite  10,000  metres 
[6  miles].  On  the  other  hand,  the  highest  terres- 
trial mountain,  Mt.  Everest,  does  not  reach  a 
height  of  9000  metres  [5.6  miles].  The  protuber- 
ances and  the  hollows  are  thus  very  small  in 
comparison  with  the  dimensions  of  the  Earth, 
although  they  appear  to  us  very  considerable. 
The  greatest  heights  and  depths  of  the  surface 
are  only  TQ-O  part  of  the  radius,  that  is  to  say, 
scarcely  oVo  part  of  the  diameter,  of  the  Earth. 
If  they  are  to  be  represented,  exactly  to  scale,  on 
a  relief  globe,  we  must  take  a  globe  one  and  a 
half  metres  [4  ft.  n  in.]  in  diameter.  Even 
then  Mt.  Everest,  with  its  8800  metres  [5.6  miles] 
on  the  one  hand,  and  the  great  oceanic  hollow 
in  the  Pacific,  9750  metres  [6  miles]  in  depth,  on 
the  other  hand,  will  be  represented  by  a  height 
and  depression  respectively  of  only  about  a 
millimetre  [.039  in.]. 

The  familiar  comparison  of  the  Earth's  irregu- 
larities to  the  wrinkles  on  the  skin  of  an  orange 
errs,  therefore,  on  the  side  of  exaggeration  of  the 
former.  The  Earth's  relief  is  very  much  less, 
relatively,  than  that  of  the  orange. 

There  is  a  special  science,  Geodesy,  the  object 
of  which  is  the  exact  measurement  of  the  Earth 


60  The  Earth 

and  the  determination  of  its  form.  If  the  Earth 
be  spherical,  an  arc  of  the  meridian  joining  any 
two  points  upon  its  surface,  separated  by  a  fixed 
number  of  degrees  of  latitude,  would  have  always 
the  same  length,  whether  near  the  pole  or  the 
equator.  If,  on  the  contrary,  the  Earth  be  ellip- 
soidal, an  arc  of  the  same  number  of  degrees  will 
be  longer  near  the  pole,  where  the  surface  is  flat- 
tened and  the  radius  of  the  curvature  consequently 
greater,  than  near  the  equator  where  the  radius 
of  curvature  is  smaller. 

The  measurement  of  meridional  arcs  is  of  such 
extreme  importance  that  the  civilised  peoples 
have  combined  together  to  form  an  International 
Geodetic  Association,  which  meets  every  three 
years  in  a  different  capital,  for  the  purpose  of 
examining  the  results  attained  and  settling  the 
programme  of  new  researches  to  be  made,  the  new 
arcs  to  be  measured.  This  association  has  also 
collected  values  of  the  intensity  of  gravitation 
which  enable  us,  by  the  difference  of  the  attrac- 
tion exerted  on  a  pendulum  in  making  it  oscillate 
more  or  less  quickly,  to  determine  the  law  by 
which  attraction  varies  according  to  the  distance 
from  the  centre  of  attraction.  Hence  we  have  a 
second  way  of  measuring  the  flattening  of  our 
globe. 


Form  and  Mass  of  tKe  EartK         6 1 

In  1799,  the  French  astronomers,  basing  their 
calculations  on  the  results  of  work  previously 
done  by  Bouguer  and  La  Condamine  in  Peru, 
Maupertuis  in  Lapland,  and  Picard  and  Cassini 
in  France,  found  that  the  entire  circumference  of 
the  Earth  should  contain  20,522,960  toises,  a  toise 
of  six  feet  being  the  legal  unit  at  that  time  in 
Paris.  In  taking  as  a  unit  a  length  of  io.ooVooo 
part  of  the  circumference,  they  thought  they 
would  obtain  one  which  would  be  approximately 
half  a  toise  and  would  accordingly  not  introduce 
much  confusion  into  current  commercial  affairs, 
at  the  same  time  having  the  advantage  of  being 
a  natural  unit  of  length.  This  unit,  the  metre 
[39-37o,ii3  in.],  became  the  foundation  of  the 
metrical  system  of  weights  and  measures,  which  is 
now  adopted  by  all  civilised  countries. r  The  metre 
is  preserved  as  the  length  of  a  bar  of  platinum  at 
o°C.  [32°  F.]  deposited  in  the  archives  in  Paris; 
copies  of  this  have  been  made  under  the  auspices 
of  the  International  Bureau  of  Weights  and  Meas- 
ures, which  is  established  at  Sevres  and  are  kept 
by  each  of  the  countries  adopting  the  system. 

The  metre  standard  was  the  result  of  the  col- 
lection of  geodetic  measurements  that  had  been 

1  In  Great  Britain  and  the  United  States  the  metric  system 
is,  unfortunately,  in  use  only  for  scientific  purposes. 


62  The  Eafth 

made  up  to  the  period  of  the  beginning  of  the 
nineteenth  century.  At  that  time,  the  flattening 
of  the  Earth  was  taken  as  sio-  part.  During  the 
last  century,  more  exact  measures  of  meridional  arcs 
have  been  perfected  and  multiplied,  and  it  is  now 
established  beyond  doubt  that  the  terrestrial  flat- 
tening is  2-Jr  part,  expressing  the  denominator 
by  the  nearest  unit.  This  being  so,  the  metre 
standard  is  somewhat  too  short,  viz.,  by  about 
t  of  a  millimetre  [.007,874  in.]. 

Scientists  in  general  have  decided  that  it  would 
be  useless,  as  this  difference  is  so  small,  to  under- 
take again  the  long  and  tedious  experiments  which 
were  necessary  for  the  establishment  of  the  original 
standard ;  the  actual  metre,  as  used  internationally, 
is  therefore  denned  as  the  length  at  o°  C.  [32°  F.] 
of  the  particular  bar  of  platinum  above  described. 
This  decision  is  both  fortunate  and  wise,  for  be- 
sides avoiding  much  heavy  work,  there  is  a  second 
reason  for  not  attempting  a  standard  exactly 
based  on  the  Earth's  magnitude.  As  will  be 
shown  in  the  course  of  the  present  work,  nothing 
in  connection  with  the  Earth  is  constant,  and  con- 
sequently it  would  be  necessary  to  be  always 
altering,  by  a  few  microns,1  the  absolute  value 
of  the  unit  of  length. 

1  A  micron  is  TOGO  of  a  millimetre  =  .00003937  inch. — Trans. 


Form  and  Mass  of  tHe  EartK         63 

Nevertheless,  as  every  material  thing  is  not 
everlasting,  but  perishable,  the  bar  of  platinum 
constituting  the  standard  metre  is  not  indestruct- 
ible; neither  are  the  copies  made  from  it.  So 
physicists  have  compared  the  value  of  their  unit 
of  length  with  another  unit  independent  of  matter, 
and  independent  even  of  the  Earth's  dimensions. 
This  new  unit  is  the  length  of  a  wave  of  light  of  a 
particular  colour,  measured  in  vacuo,  that  is  to 
say,  the  distance  which  separates  any  two  con- 
secutive crests  of  the  waves  generated  in  the  ether 
by  the  vibratory  movements  which  constitute 
light.  To  the  American  physicist,  Michelson  of 
Chicago,  is  due  the  credit  of  having  first  attempted 
this  comparison.  He  was  successful  and  found, 
after  much  difficult  work,  wonderful  because  of 
its  precision  and  the  perseverance  necessary  for 
its  completion,  that  a  metre  contained  1,553,163.5 
times  the  length  of  the  wave  of  the  red  cadmium 
light,  and  2,083,372  times  the  blue  radiation  from 
the  same  metal.  Thus  we  are  no  longer  entirely 
dependent  on  the  standard  metre  bar  of  platinum, 
and  our  unit  is  obtainable  from  one  which  is  inde- 
structible, as  it  will  exist  as  long  as  light  itself. 
Clerk  Maxwell  already  realised  the  importance  of 
a  unit  independent  of  the  Earth's  size  when  he 
wrote  in  the  preface  of  his  famous  treatise  on 


64  The  Earth 

electricity :  "Such  a  standard  would  be  independent 
of  any  changes  in  the  dimensions  of  the  Earth, 
and  should  be  adopted  by  those  who  expect 
their  writings  to  be  more  permanent  than  that 
body." 

Geodetic  measures  afford  us  exact  knowledge 
of  the  Earth's  size,  in  contradistinction  to  the 
experiment  mentioned  at  the  beginning  of  this 
chapter  which  gives  only  an  approximate  result. 
Let  us  begin  by  understanding  that  in  the  deter- 
mination of  the  Earth's  form  no  account  is  taken 
of  the  continental  protuberances  or  the  oceanic 
hollows.  We  will  suppose  the  line  of  sea-level 
to  be  prolonged  under  the  continents  and  the 
imaginary  surface  thus  produced,  called  the  geoid, 
is  that  whose  form  we  will  endeavour  to  determine. 
The  fluidity  of  the  oceans  causes  them  to  obey 
the  laws  of  attraction  and  centrifugal  force,  and 
mechanics  shows  that  such  a  surface  can  only  take 
a  flattened  figure,  which  is  that  of  an  ellipsoid  of 
revolution. 

This  ideal  surface  is  not  entirely  of  theoretical 
use;  the  operation  of  levelling  consists  precisely 
in  finding  the  height  of  each  point  of  the  land  sur- 
face above  it,  that  is  to  say  it  resolves  itself  into 
finding  the  distance  between  any  point  on  the 
Earth's  surface  and  the  surface  of  the  sea  pro- 


Form  and  Mass  of  tHe  EartH.         65 

longed  in  imagination  beneath  it.  The  mining 
engineer  Lallemand  has  carried  this  work  to  an 
unlooked-for  degree  of  precision.  Now,  as  will 
be  seen  later,  the  mean  altitude  of  the  continents 
is  only  700  metres  [3000  ft.],  which  is  scarcely 
more  than  10.000  part  of  the  Earth's  radius. 
Also,  the  slope  of  the  land  towards  the  sea  is  in 
general  very  slight;  the  courses  of  the  large  rivers 
give  some  idea  of  it.  It  is,  therefore,  quite  legiti- 
mate to  take  the  form  of  the  geoid  as  that  of  the 
Earth  itself,  except  when  it  is  desired  to  determine 
the  altitude  of  any  land  or  the  depth  of  the  oceanic 
abysses  or  when  we  wish  to  investigate  the  local 
anomalies  of  the  surface. 

The  results  of  these  measures  discussed  with 
so  much  care  by  the  German  geodesist  Helmert 
have  led  to  the  adoption  of  the  following  values. 
The  semi-major  axis  of  the  terrestrial  ellipsoid, 
that  is  to  say  the  radius  of  the  terrestrial  equator, 
is  6,377,857  metres  [3963.125  miles]  long.  The 
semi-minor  axis,  which  is  the  distance  from  one  of 
the  poles  to  the  Earth's  centre,  is  6,356,606  metres 
[3949-92  miles]  in  length.  The  most  probable 
value  of  the  flattening  is  «rJr.  The  Earth's  cir- 
cumference at  the  equator  is  40,073,  351  metres 
[24,900  miles]. 

On  account  of  the  flattening,  the  North  Pole 


66  TKe  EartK 

is  about  20  kilometres  [13  miles]  nearer  to  the 
Earth's  centre  than  is  the  equator. 

Since  we  know  the  dimensions  of  the  terrestrial 
ellipsoid  we  can  determine  its  surface  and  its 
volume.  The  total  extent  of  the  Earth's  surface 
is  510,082,000  square  kilometres  [197,000,000 
sq.  miles].  Of  this,  the  continents  and  islands 
occupy  145,000,000  [56,000,000  sq.  miles]  and  the 
oceans  and  seas  365,000,000  [141,000,000,  sq.  miles]. 
There  is  not,  as  will  be  seen,  an  equal  division  into 
land  and  water;  the  latter  occupies  two  and  a  half 
times  as  great  a  surface  as  the  former. 

Not  only  is  there  not  an  equal  division  as  re- 
gards the  whole  Earth,  but  the  distribution  of  the 
land  is  very  different  in  the  two  hemispheres. 
If  we  take  as  centre  of  a  hemisphere  the  little 
lie  Dumet,  which  lies  near  the  mouth  of  the  Vilaine, 
in  Southern  Brittany,  or,  in  other  words,  if  we 
place  ourselves  sufficiently  far  from  a  terrestrial 
globe,  arranging  matters  so  that  a  line  from  the 
eye  to  the  centre  of  the  globe  passes  through  this 
little  island,  the  hemisphere  that  will  be  visible 
contains  exactly  as  much  land  surface  as  water 
surface  (Fig.  6).  On  the  other  hand,  the  hemis- 
phere opposite  to  this  one  will  be  essentially  a 
marine  hemisphere,  containing,  as  it  does,  nine 
times  more  water  than  land  surface.  Our  globe 


Form  and  Mass  of  tKe  EartH          67 

thus  possesses  a  land  hemisphere  and  a  marine 
hemisphere.  Without  being  so  characteristic  as 
the  foregoing,  the  aspect  of  a  terrestrial  globe, 
when  the  eye  is  placed  in  front  of  that  point  of  the 
equator  which  occupies  the  middle  of  the  Pacific, 
is  also  very  instructive.  An  immense  stretch  of 
water,  the  Pacific  Ocean,  is  seen  extending  over 


FIG.  6. — Land  Hemisphere  and  Water  Hemisphere 
(Taking  as  a  pole  the  lie  Dumet.) 

nearly  a  whole  hemisphere,  while  the  opposite  one 
contains  the  greater  portion  of  two  continents. 

Every  point  of  the  Earth  has  what  is  called  its 
antipode,  that  is  to  say  the  point  which  is  dia- 
metrically opposed  to  it  in  the  other  hemisphere. 
Now,  only  2^0  of  the  land  surface  has  an  element  of 
land  as  antipodal  point ;  the  other  ^  have  a  point 
of  the  sea  surface  opposite  to  them.  From  this 
there  follows  a  general  law,  that  of  the  dia- 
metrical opposition  of  the  continents  and  the  seas. 


68  TKe  Earth 

As  Lapparent  has  expressed  it,  there  are  nineteen 
chances  to  one  that  any  element  of  the  land  surface 
will  have  as  antipodal  point  a  part  of  the  Earth's 
surface  which  is  covered  by  the  sea. 

This  diametrical  opposition  of  land  and  water 
results  from  the  tendency  exhibited  by  the  ter- 
restrial crust  to  take  a  tetrahedral  form  at  the 
time  of  its  solidification.  This  form  is  that  of  a 
pyramid  with  three  equilateral  faces,  each  apex  of 
which  is  opposite  to  a  face  and  vice  versa.  The 
faces  of  the  tetrahedron  are  represented  by  the 
oceans  (see  Fig.  4,  p.  32,  and  Fig.  56,  p.  54),  and 
the  apices  correspond  to  the  emergent  land;  con- 
sequently there  is  an  opposition  between  the 
continents  and  oceans  on  the  Earth's  surface. 
The  examination  of  a  map  of  the  world  will  con- 
firm this.  The  land  surface,  which  is  in  great 
excess  in  the  Northern  Hemisphere,  may  be  sub- 
divided into  three  chief  masses :  the  European  con- 
tinent, the  Asiatic  continent  with  its  Australian 
prolongation,  and  the  American  continent.  There 
are  also  three  chief  oceans,  the  Atlantic,  Pacific, 
and  Indian  oceans. 

Furthermore,  polar  expeditions  have  furnished 
additional  proof  of  the  diametrical  opposition; 
around  the  North  Pole  is  the  Arctic  Ocean  with 
depths  of  more  than  3000  metres  [9800  ft.],  while 


Form  and  Mass  of  tKe  EartK         69 

about  the  South  Pole  there  exists,  on  the  contrary, 
an  antarctic  continent  of  considerable  elevation 
and  an  extent  of  the  same  order  as  that  of  Europe. 
The  distribution  of  lands  and  seas  may,  therefore, 
be  considered  to  conform  to  the  law  of  diametrical 
opposition.  In  the  course  of  Chapter  VII., 
when  describing  seismic  phenomena,  we  shall 
return  in  fuller  detail  to  the  tetrahedral  theory. 

Having  studied  the  Earth's  surface  we  shall 
now  consider  the  facts  relative  to  its  volume. 

The  volume  of  the  ellipsoid  is  1,083,260  millions 
of  cubic  kilometres  [260,000,000,000  cubic  miles], 
and,  in  order  to  give  some  material  idea  of  what 
such  volume  is,  we  shall  state  that  it  would  require 
340  times  the  volume  of  the  Great  Pyramid  of 
Egypt  to  equal  one  cubic  kilometre.  The  Earth 
is  not  truly  spherical,  but  is  flattened  at  the  poles 
and  bulges  at  the  equator;  the  volume  of  this 
bulge  is  the  TTT  part  of  the  total  volume  of  the 
Earth. 

There  are,  also,  other  volumes  of  interest  to  us 
besides  that  of  the  globe  taken  as  a  whole.  We 
may  try  to  evaluate  the  volume  of  all  the  land 
which  rises  above  sea-level  and  also  the  total 
volume  of  all  the  water  contained  in  the  oceans. 
We  have  already  noted  the  great  inequality  be- 
tween the  continental  and  marine  surfaces,  and 


70  The  EartH 

this  inequality  becomes  still  greater  when  we  come 
to  examine  the  relative  volumes. 

The  total  volume  of  the  continents  above  sea- 
level  is  about  100  million  cubic  kilometres  and  the 
volume  of  the  water  contained  in  all  the  seas  of 
the  globe  is  1300  million  cubic  kilometres.  The 
volume  of  the  ocean  is  therefore  thirteen  times 
greater  than  that  of  the  continents. 

As  a  result  of  the  precise  measures  of  altitude 
and  depth,  an  old  and  false  idea  of  ancient  geo- 
graphers falls  to  the  ground.  They  believed  that* 
if  the  water  of  the  seas  were  removed  and  the 
continents  razed  down  to  the  sea-level,  the  debris 
of  the  latter  would  just  fill  up  the  oceanic  hollows. 
Nothing  could  be  more  untrue.  If  we  imagine  a 
somewhat  different  operation,  that  of  all  the  emer- 
gent land  being  formed  into  a  shell  of  uniform 
thickness,  having  the  same  contours  as  those  of 
the  actual  continents,  the  thickness  would  be  only 
700  metres  [3000  ft.].  If,  however,  the  same  thing 
were  done  with  the  bed  of  the  seas,  so  that  they 
were  all  of  uniform  depth,  the  contours  remaining 
as  at  present,  the  resulting  depth  of  water  would 
be  about  3550  metres  [2.2  miles].  As  this  depth  is 
spread  over  a  surface  two  and  a  half  times  greater 
than  that  of  the  land,  the  mean  altitude  of  which 
is  only  a  fifth  part  of  the  mean  depth  of  the  seas, 


Form  and  Mass  of  tHe  EartH         71 

the  difference  between  the  two  volumes  is  very 
marked. 

We  have  next  to  consider  how  the  protuberances 
and  hollows  which  form  the  relief  of  the  terrestrial 
crust  are  distributed  on  the  land  surface  and  the 
bed  of  the  sea.  The  imagination  of  the  ancient 
poets  depicted  the  ocean  as  a  gulf  whose  depth, 
which  was  almost  infinite,  increased  rapidly  as  the 
shores  were  left,  ending  in  abysses  inhabited  by 
frightful  monsters.  Now,  soundings  have  been 
so  universally  made  that  Prince  Albert  of  Monaco 
has  been  able  to  draw  up  a  topographical  map  of 
the  bed  of  the  oceans  just  as,  and  in  the  same  way, 
the  laborious  work  of  the  ordnance  officers  of  all 
countries  have  enabled  detailed  maps  of  the  relief 
of  the  land  surface  to  be  made. 

In  examining  these  maps,  it  is  seen  that  a  con- 
tinent is  not  a  kind  of  regular  dome  whose  summit 
occupies  the  middle  part,  and  also  that  an  ocean 
is  not  a  kind  of  funnel  whose  sides  converge  to- 
wards a  central  hole.  On  the  contrary,  almost 
all  the  important  mountainous  masses  are  for 
the  greater  part  of  their  length  situated  on  the 
margin  of  the  oceans;  for  example,  the  Andes  along 
the  Pacific  coast,  the  Alps  along  the  shore  of  the 
Mediterranean,  and  the  Scandinavian  mountains 
close  to  the  North  Sea.  It  is  just  the  same  with 


72  The  EartK 

regard  to  the  marine  hollows.  The  greatest 
depths  are  not  to  be  found  at  the  centre  of  the 
seas;  thus  the  chief  depths  of  the  Pacific  are  situ- 
ated in  its  western  part,  where  the  sounding  line 
has  descended  in  several  parts  below  9000  metres 
[5.6  miles],  and  the  same  holds  in  the  case  of  the 
Atlantic,  which  is  deeper  at  its  edges  than  in  the 
middle,  where  a  long  submarine  ridge  exists. 

If  the  sea  were  to  disappear  completely,  leaving 
uncovered  that  part  of  the  Earth's  crust  which 
forms  its  bed,  the  attentive  observer  would  not 
notice  any  special  character  suggesting  the  situa- 
tion of  the  former  oceans.  The  bed  of  the  sea 
has  a  relief  like  that  of  the  land  surface,  and  if  the 
higher  parts  of  the  latter  are  more  precipitous  than 
the  more  rounded  submarine  summits,  it  is  because 
of  the  erosive  action  which  exterior  agents  exercise 
on  the  emergent  surface,  while  the  oceans  protect 
the  irregularities  covered  by  their  waters. 

Nevertheless,  certain  general  tendencies  may 
be  observed  on  the  relief  globe  which  would  result 
if  the  Earth  were  deprived  of  its  seas,  and  these 
tendencies  are  so  general  that  they  may  be 
formulated  into  laws.  Thus,  the  accidents  of  the 
relief  do  not. have  two  sides  symmetrically  inclined 
towards  the  lower  surrounding  regions;  they  are, 
on  the  contrary,  unsymmetrical.  In  the  case  of 


Form  and  Mass  of  tHe  EartH         73 

the  slopes  of  an  oceanic  hollow,  or  the  flanks  of  a 
chain  of  mountains,  one  side  is  almost  always 
abrupt  and  the  other  a  gentle  declivity.  Fur- 
thermore, in  most  cases,  the  abrupt  slope  of  a 
mountain  chain  bordering  an  ocean  is  continuous 
with  the  equally  abrupt  side  of  a  hollow  in  the 
ocean  bed,  so  that  the  mountain  thus  seems  to  dip 
sharply  into  the  sea  while  the  slope  of  its  other 
side  stretches  for  a  long  distance  with  a  gentle 
fall.  The  Andes  constitute  the  most  striking  case 
of  this;  their  summits  are  6000  and  7000  metres 
[3.7  to  4.4  miles]  high,  and  the  ridge  falls  sharply 
to  the  Pacific,  its  steep  slope  being  continued  under 
the  sea  by  a  long  hollow  having  depths  of  6000, 
7000,  and  8000  metres  [3.7,  4.4,  and  5  miles], 
while  the  opposite  side  of  the  range  slopes  gently 
towards  the  Atlantic  until  it  is  lost  in  the  Argen- 
tine Pampas. 

As  has  been  neatly  expressed  by  Lapparent, 
the  crust  of  the  Earth  as  a  whole  resembles  an  old 
patchwork  whose  parts  have  shifted  with  respect 
to  each  other,  and  in  the  places  where  the  great 
folds  are  produced,  such  as  that  which  gave  rise 
to  the  Andes,  it  is  as  if  the  crust,  being  no  longer 
sustained  below  at  all  its  parts  by  the  contracting 
nucleus,  had  acted  as  does  a  piece  of  material, 
which  is  too  large,  that  is  to  say  formed  a  fold, 


74  The  EartH 

the  sides  of  which  after  a  sudden  descent  merge 
into  the  general  level. 

There  is  one  characteristic  fact:  these  folds,  the 
irregularities  of  the  surface,  seem  to  run  in  lines. 
A  number  of  such  alignments  may  be  seen  on  the 
map  of  the  world :  the  Ural  Mountains,  the  Andes, 
the  oceanic  hollows  of  Polynesia.  The  islands  which 
fringe  the  Pacific  clearly  demonstrate  this  tendency. 

Knowing  its  dimensions,  it  remains  to  deal  now 
with  the  Earth's  mass.  It  is  to  be  noted  that 
we  speak  of  the  mass  and  not  the  weight  of  the 
Earth.  The  idea  of  mass  requires  to  be  carefully 
explained  and  is  quite  different  from  that  of  weight. 
The  mass  of  a  body  is  the  quantity  of  matter  it 
contains,  whatever  the  exterior  form  under  which 
the  said  matter  is  revealed  to  us.  If  the  body  be 
at  rest,  this  quantity  of  matter  remains  in  that 
state  by  reason  of  its  inertia;  if,  on  the  other  hand, 
a  given  force  acts  on  the  body  tending  to  displace 
it  and  to  impress  any  movement  whatever  upon  it, 
the  resulting  motion  will  the  less  readily  take 
place  in  proportion  as  the  mass  is  greater.  Poisson 
has  well  stated  this  idea  of  mass  in  saying:  Mass 
is  the  coefficient  of  resistance  to  motion. 

The  mass  of  a  given  body  is  thus  a  constant 
quantity.  Its  weight,  on  the  contrary,  which  is 
the  force  with  which  it  is  attracted  by  the  Earth, 


Form  and  Mass  of  tHe  EartH         75 

varies  in  proportion  as  the  body  is  moved  either 
horizontally,  from  one  place  to  another,  or  verti- 
cally. It  should  be  noted  that,  as  the  attraction 
of  the  Earth  exercised  upon  objects  at  or  above  its 
surface  is  the  cause  of  weight,  the  Earth  itself,  as 
a  whole,  can  have  no  weight  in  the  strict  sense  of 
the  word.  It  cannot  attract  itself.  The  Earth 
has  weight  with  respect  to  the  Sun,  but  has  no 
weight  in  the  sense  in  which  the  word  is  used  for 
the  bodies  on  its  own  surface.  The  evaluation  of 
its  mass  is  therefore  the  only  question  concerning 
us. 

It  will  be  remembered  that  the  mass  of  a  gramme 
[15.432  grains]  has  been  taken  as  a  scientific  unit. 
Now,  forces  cannot  be  expressed  in  grammes  but 
have  to  be  referred  to  a  special  unit,  called  the 
dyne,  a  dyne  being  the  force  which  when  applied 
to  a  body  of  mass  one  gramme  gives  to  it  a  uniform 
acceleration  of  one  centimetre  [.3937  inch]  per 
second. 

In  order  to  measure  the  total  mass  of  the  Earth, 
an  experiment,  based  on  the  Newtonian  law  of 
attraction,  is  made.  Two  bodies  are  chosen,  one 
movable  and  of  small  mass,  the  other  fixed  and  of 
considerable  mass.  It  is  essential  to  use  methods 
of  measurement  which  are  sufficiently  sensitive 
to  show  the  displacement  which  the  small  body 


76  The  Earth 

suffers  under  the  attractive  force  of  the  known 
mass  of  the  greater  one.  The  displacement  being 
known,  we  are  able  to  exert  an  opposing  force  on 
the  small  body  which  neutralises  it  and  whose 
measure  gives  us  that  of  the  attractive  force. 

We  may  use  as  the  large  attracting  mass  either 
a  natural  mass  such  as  that  of  a  mountain,  or  a 
selected  mass  for  use  in  the  laboratory.  There  are 
thus  two  distinct  methods,  the  geographical  and 
the  physical  one. 

The  attraction  exercised  by  a  mountain  on  a 
body  of  small  mass  placed  in  its  neighbourhood 
can  be  measured  in  two  quite  different  ways. 
The  first  is  by  the  observation  of  the  swing  of  a 
pendulum  upon  the  summit  of  the  mountain. 
Since  the  pendulum  is  thus  at  an  elevation  the 
action  of  gravity  upon  it  is  feebler  and  the  amount 
of  such  action  may  easily  be  calculated  when  the 
height  of  the  summit  is  known.  Now,  it  is  found 
that  the  swing  thus  calculated  is  not  the  same  as 
the  swing  actually  observed;  there  is  a  perturbing 
action  due  to  the  presence  of  the  mountain,  which 
acts  contrary  to  the  diminution  of  gravity  due  to 
altitude  alone.  The  difference  between  the  cal- 
culated and  observed  times  of  swing  thus  enables 
us  to  determine  the  attractive  action  of  the 
mountain ;  this  is  the  dynamic  method. 


Form  and  Mass  of  the  Earth         77 

The  second  is  a  static  method.  Let  us  suppose 
that  the  density  of  the  mountain  is  known  by 
reason  of  the  work  of  geologists,  and  the  position 
of  its  centre  of  gravity  exactly  determined  by 
precise  topographical  operations.  If  we  take  up 
a  position  to  the  north  of  the  mountain,  and  there 
set  up  a  plumb  line,  it  will  be  slightly  deviated, 
the  suspended  mass  being  attracted  towards  the 
centre  of  gravity  of  the  mountain  by  the  mass 
of  the  latter.  Produced  to  intersect  the  celestial 
sphere,  the  plumb  line  would  not  meet  it  at  the 
same  point  as  if  the  mountain  did  not  exist,  but 
in  one  more  to  the  north.  Similarly  a  plumb  line 
placed  at  the  south  of  the  mountain  would  also 
be  attracted  by  it,  and  the  line  if  prolonged  would 
meet  the  celestial  sphere  at  a  point  to  the  south 
of  that  at  which  it  would  have  done  so  if  the 
mountain  had  not  attracted  the  suspended  mass. 

In  other  words, 
these  two  plumb 
lines  would  be  di- 
rected, if  the  moun- 
tain did  not  exist, 
along  tWO  Of  the  FIG.  7. -Deviation  of  a  Plumb  Line 

by  a  Mountain, 
radii  of  the   Earth 

and  so  would  intersect  at  its  centre.     In  the  pres- 
ence of  the  mountain  both  are  deviated  towards 


78  The  Earth 

each  other  and,  if  prolonged,  would  intersect  at  a 
point  nearer  the  Earth's  surface  than  its  centre 
(Fig  7).  If  there  were  no  deviation,  the  angle 
made  by  the  two  plumb  lines  produced,  would 
be  equal  to  that  between  the  verticals  at  the  given 
points  of  observation,  that  is  to  say,  equal  to  the 
actual  difference  of  latitude  between  the  two 
stations.  When  deviation  is  caused  by  the  moun- 
tain the  two  lines  make  an  angle  greater  than  the 
latitude  difference.  If  this  angle  can  be  deter- 
mined we  should  be  able  to  calculate  the  attrac- 
tion exerted  by  the  mountain  on  the  small  sus- 
pended masses. 

Now,  this  angle  may  be  obtained  astronomically. 
All  that  is  necessary  is  to  find,  by  observation  of  a 
star,  the  altitude  of  the  pole  above  the  plane  of  the 
horizon  for  each  of  the  two  stations,  the  horizon 
being  defined  as  the  plane  exactly  perpendicular 
to  the  plumb  line  in  question.  The  real  difference 
of  latitude,  on  the  other  hand,  may  be  measured 
by  finding  the  distance  between  the  two  places  by 
means  of  topographical  operations.  The  observed, 
or  apparent,  difference  may  thus  be  compared  with 
the  real  one  and  hence  twice  the  angle  of  the 
required  deviation,  caused  by  the  mountain, 
deduced. 

Such  is  the  principle  of  the  geographical  method. 


Form  and  Mass  of  tHe  EartH         79 

It  is  obvious  that  any  other  natural  mass  may  be 
made  use  of  as  the  attracting  body,  provided  its 
mass  can  be  exactly  determined.  This  is,  however, 
the  weak  point  in  the  method,  at  least  as  far  as 
mountains  are  concerned;  their  mean  density  is 
always  uncertain,  for  the  disposition  of  the  rocks 
and  minerals  composing  them  is  not  precisely 
known.  As  a  result  there  is  an  uncertainty  not 
only  as  to  the  value  of  the  attracting  mass,  but 
also  concerning  the  exact  position  of  the  centre 
of  gravity.  Nevertheless  such  methods  were  the 
first  ones  made  use  of ;  Bouguer  and  La  Condamine, 
in  1736,  determined  the  deviation  exerted  on  the 
plumb  line  by  the  mass  of  Chimborazo  in  Peru, 
and  found  the  sum  of  the  angular  deviations  on  the 
north  and  south  of  the  mountain  to  be  19  seconds 
of  arc.  In  1774,  Maskelyne  repeated  this  experi- 
ment in  Scotland,  studying  the  deviation  from  the 
vertical  caused  by  the  mountain  Schiehallion. 
Two  stations  were  chosen  to  the  north  and  south 
of  the  mountain,  respectively,  and  topographical 
operation  gave  43  seconds  as  the  real  difference 
of  latitude  of  the  two  places;  the  same  difference 
measured  astronomically  was  found  to  be  54.5 
seconds.  The  discrepancy  therefore  was  11.5 
seconds,  due  to  the  sum  of  the  angular  deviations 
exerted  on  the  two  plumb  lines  by  the  attraction 


8o  The  Earth 

of  the  mountain.  At  the  same  time,  the  geologist 
Hutton  had  studied  the  composition  of  the  mount- 
ains and  had  estimated  its  volume  as  precisely 
as  possible;  this  operation  alone  took  more  than 
three  years. 

As  a  direct  result  of  such  an  experiment,  we 
find  the  intensity  with  which  a  mountain  of  known 
mass  deviates  a  suspended  mass  from  the  vertical, 
at  a  distance  from  its  centre  of  gravity.  This 
small  mass  may  be  weighed  on  a  balance;  we 
then  know  with  what  intensity  the  earth  (sup- 
posedly spherical)  attracts  towards  its  centre  a 
mass  situated  at  a  point  of  its  surface,  at  a  dis- 
tance from  the  centre  equal  to  the  radius  of  the 
Earth. 

The  ratio  of  the  values  of  the  attractive  forces 
gives  the  ratio  of  the  attracting  masses  in  the  two 
cases.  The  value  of  the  Earth's  mass  may,  there- 
fore, be  deduced  from  that  of  the  mountain.  Also, 
mass  is  the  product  of  volume  and  density,  and 
the  Earth's  volume  is  known  from  the  geodetic 
operations  which  give  its  dimensions.  Therefore, 
if  we  divide  the  mass  by  the  volume,  we  obtain 
the  mean  density  of  the  globe.  This  density  may 
thus  be  imagined :  Let  us  suppose  the  whole  Earth 
to  be  ground  to  powder  in  a  gigantic  mortar  and 
the  powder  subsequently  mixed  and  stirred  in- 


Form  and  Mass  of  tKe  EartK          8 1 

timately  together;  a  substance  would  be  obtained 
whose  density,  that  is  to  say  mass  per  unit  volume, 
would  be  the  mean  density  of  the  Earth. 

Experiments  analogous  to  those  of  Maskelyne 
have  shown  that  this  mean  density  is  approxi- 
mately equal  to  5.5.* 

We  shall  see  later  how  important  this  result  is 
with  reference  to  our  knowledge  of  the  internal 
structure  of  our  globe. 

The  physical  methods  for  the  determination  of 
the  Earth's  density  are  susceptible  of  a  much 
higher  precision,  and  can  be  carried  out  in  a  labo- 
ratory. To  Cavendish  is  due  the  credit  of  having 
designed  the  first  apparatus  of  this  kind  and 
having  made,  as  Joseph  Bertrand  has  said,  "a 
balance  to  weigh  the  Earth." 

The  attractive  forces  exercised  by  one  portion 
of  matter  on  another  are  very  feeble,  and  the 
reason  the  force  of  gravity  is  so  great  is  only 
because  the  mass  of  the  Earth  is  relatively  so 
large.  We  shall  now  see  how  the  value  of  the 
attractive  force  between  two  masses  each  equal  to 
unity  can  be  found.  Since  the  force  is  very  slight, 
it  is  necessary  to  make  use  of  an  opposing  force, 
also  very  small,  to  counterbalance  it.  For  this 

1  The  standard  of  density  ( =  i)  is  that  of  water  at  a  temperature 
of  4°  C.  [39.2°  F.1.—E0. 


82 


The  Earth 


purpose  Cavendish  chose  the  torsion  or  twist  of 

a  long,  fine  wire  (Fig.  8). 

He  fixed  two  small  lead- 
en balls  of  known  equal 
mass  at  the  extremities  of 
a  very  light  lever  which 
was  suspended  at  its  mid- 
point by  a  silver  wire.  The 
lever,  therefore,  pointed  in 
a  fixed  direction  dependent 
on  the  position  in  which 
the  wire  was  free  from 

FiG^.-Cavendish's  Ex-  st rain  •       Tw°  larSe  fixed 

periment.  leaden   balls  of  known 

mass  were  introduced,  as  shown,  to  the  right 
of  the  movable  ones  (the  right  being  that  on 
the  right  hand  when  looking  from  either  of  the 
small  balls  towards  the  wire).  Each  of  the  large 
balls  attracted  the  small  one  near  it,  and  their 
added  effect  tended  to  turn  the  lever  so  as  to 
bring  the  small  balls  up  to  them.  But  this  motion 
was  opposed  by  the  torsion  or  twist  of  the  wire, 
the  value  of  which  had  been  carefully  determined 
by  preliminary  experiments.  The  angle  through 
which  the  lever  actually  turned  before  it  was 
brought  to  rest  by  the  opposing  force  thus  served 
to  measure  the  value  of  the  force  which  balances 


Form  and  Mass  of  tKe  EartK          83 

the  attraction  between  the  balls,  and,  this  being 
exerted  between  known  masses  at  known  distances, 
was  thus  fully  established  by  experiment.  The 
whole  apparatus  was  placed  tinder  cover  so  as  to 
guard  against  disturbing  air  currents. 

Besides  the  equilibrium  method,  Cavendish 
also  employed  a  dynamical  method  which  con- 
sisted in  studying  the  oscillations  of  the  lever 
deviated  from  its  normal  position  by  the  attrac- 
tion of  the  large  balls  and  brought  back  towards 
this  position  by  the  torsion  of  the  wire,  which 
made  it  oscillate  similarly  to  a  pendulum.  This 
attraction,  compared  to  that  of  the  Earth  on  the 
seconds  pendulum,  gives  the  ratio  of  the  density 
of  lead  to  the  mean  density  of  the  Earth;  the 
average  result  of  twenty-nine  determinations  has 
given  5.48  as  the  required  mean  density,  that  of 
water  being  taken  as  unity. 

This  experiment  has  been  repeated  under  the 
most  diverse  forms;  Cavendish's  apparatus  has 
been  modified  in  accordance  with  all  the  progress 
that  has  since  been  achieved  in  the  construction 
of  physical  instruments  and  also  in  the  methods 
of  observation.  But  these  measurements,  some  of 
which  have  been  made  by  illustrious  physicists 
such  as  Baron  Eotvos,  Cornu,  Bailie,  Poynting, 
etc.,  have  only  confirmed  the  mean  value  5.5 


84  The  Earth 

found  by  Cavendish,  in  spite  of  the  greater  deli- 
cacy of  the  experiments.  This  illustrates  the 
fact,  demonstrated  by  the  history  of  all  science, 
that  the  man  who  first  discovers  a  phenomenon, 
devises  a  method  and  makes  an  apparatus  for 
measuring  it,  determines  at  once  the  true  value 
of  the  measure.  He  makes  up  for  the  instrumental 
deficiency  by  that  particular  intuition  which  con- 
stitutes the  essence  of  genius.  If  we  consider  the 
great  quantitative  determinations  of  the  laws  of 
physics,  e.  g.,  the  mechanical  equivalent  of  heat, 
the  velocity  of  light,  the  latent  heat  of  fusion  of 
ice,  etc.,  in  every  case  the  later  results,  more  and 
more  precise,  have  only  confirmed  the  figures 
obtained  by  him,  who,  in  each  case,  may  justly 
be  called  the  first. 

We,  therefore,  know  the  Earth's  mean  density, 
from  which  it  follows  that  its  mass,  compared  to 
the  mass  (not  the  weight)  of  a  kilogram,  is  6,100,- 
000,000,000,000,000,000,000  [3,716  quintillions  of 
tons}  or  as  modern  physicists  would  write  it 
6.  i  X 10 2  4  kilograms.  The  imagination  can  hardly 
picture  such  a  mass,  and  yet  it  is  a  very  small 
one  in  comparison  with  the  masses  of  some  of 
the  celestial  bodies. 

One  result  of  these  determinations  is  that  we 
can  calculate  the  force  with  which  two  known 


Form  and  Mass  of  the  EartH          85 

masses,  placed  at  a  known  distance  apart,  attract 
each  other. 

Newton 's  law  tells  us  that  any  two  bodies 
whatsoever  attract  each  other  with  a  force  directly 
proportional  to  the  product  of  their  masses  and 
inversely  proportional  to  the  square  of  the  dis- 
tance which  separates  them.  This  law  may  be 
expressed  by  a  very  simple  formula;  the  intensity 
of  the  force  is  obtained  by  multiplying  together 
the  masses  of  the  two  bodies,  expressed  in  grams 
[15.432  gr.]  and  dividing  the  product  by  the 
square  of  the  distance  apart,  expressed  in  centi- 
metres [.3937  in.],  the  whole  being  multiplied  by 
a  coefficient  which  denotes  the  proportionality.1 
This  coefficient,  which  is  the  constant  of  gravita- 
tion, is  not  an  abstract  number  but  has,  on  the 
contrary,  a  very  distinct  physical  significance. 
It  represents  the  intensity  of  the  force  of  attrac- 
tion between  two  masses,  each  of  one  gram,  a 
centimetre  apart.  The  coefficient  is  very  small; 
it  is  denoted  by  the  letter  k,  and  its  numerical 
value  equals  loo.o^o.ooo  *.  e->  k  =  6.5Xio~8. 

This  force  is  expressed  in  units  of  force,  that  is 
to  say,  in  dynes,  the  gram  being  the  unit  of  mass. 
A  dyne  is  the  force  which  gives  an  acceleration  of 

1  The  formula  of  Newton's  Law  is  f =k  -57-  where  m,m'  are 
the  masses  of  the  two  bodies  concerned  and  d  the  distance. 


86  The  EartH 

one  centimetre  per  second  to  the  mass  of  one 
gram.  If  the  force  of  gravity  instead  of  the  dyne 
acted  on  this  mass,  the  acceleration  produced  would 
be  981  centimetres  [32  ft.  2.2  in.]  per  second  at 
Paris,  as  measured  by  pendulum  experiments. 
Consequently  a  dyne  represents  the  QSist  part 
of  the  weight  of  a  gram;  it  is  a  force  of  sensibly 
the  same  power  as  that  exerted  by  the  weight  of 
a  milligram  [.oi5432gr.]. 

We  can  now  easily  calculate  the  force  between 
any  two  given  bodies.  If  we  wish,  for  example, 
to  find  that  exerted  by  a  sphere  of  lead,  10  metres 
in  diameter  [33  ft.  9.7  in.],  on  a  spherical  mass  of 
one  kilogram  [2  Ibs.  3.27  ozs.],  also  of  lead,  whose 
diameter  is  2  decimetres  [7.874  in.],  the  result  is  a 
force  which,  compared  to  the  weight  of  a  gram 
[15.432  gr.],  is  not  quite  half  a  milligram  [.00771 6  gr.]. 
It  is,  therefore,  the  enormous  mass  of  the  celestial 
bodies  to  which  the  importance  of  the  attractive 
forces  between  them  is  due.  And  the  forces  are, 
of  course,  greatly  diminished  by  reason  of  the 
vast  distances  separating  the  bodies,  especially 
as  it  is  the  square  of  the  distance  which  comes 
into  the  formula. 

The  knowledge  of  the  Earth's  mean  density, 
which  Cavendish  and  his  successors  have  shown 
to  be  equal  to  5.5,  leads  to  a  conclusion  of  the 


Form  and  Mass  of  the  EartH         87 

highest  importance  relative  to  the  constitution 
of  the  interior  of  our  globe.  The  density  of  the 
rocky  strata  which  constitute  the  Earth's  super- 
ficial crust  never  greatly  exceeds  the  mean  value 
2.5.  In  order,  therefore,  for  the  Earth  to  have  a 
mean  density  of  5.5,  the  density  of  the  interior 
must  have  a  much  greater  value  in  order  to  com- 
pensate for  the  relative  lightness  of  the  outer 
layers. 

Roche  and  Wiechert  have  made  a  study  of  this 
question  and  have  deduced  the  law  which  bears 
their  joint  names,  the  law  of  Roche-Wiechert. 
The  Earth  is  composed  of  a  nucleus  of  which  the 
density  is  about  10  and  whose  diameter  is  eight- 
tenths  of  that  of  the  whole  globe;  this  nucleus  is 
surrounded  by  a  spherical  layer  of  lesser  density. 
As  only  metals  have  densities  as  high  as  10,  we 
must  suppose  the  central  mass  of  the  Earth  to 
be  composed  of  metallic  matter.  Furthermore 
the  electrical  and  magnetic  phenomena  of  which 
the  Earth  is  the  seat  and  the  constitution  of  the 
lavas  thrown  out  from  volcanoes  show  that  this 
metallic  nucleus  is  largely  composed  of  iron. 

We  have  next  to  consider  in  what  state  such 
dense  metallic  matter  exists  in  the  Earth's  central 
portion.  Every  time  that  we  penetrate  into  the 
Earth's  interior,  for  example,  on  descending  the 


88  The  Earth 

shaft  of  a  very  deep  mine,  we  find  a  continuous 
increase  of  temperature  as  the  depth  below  the 
surface  increases.  Such  increase  is  usually  pro- 
portional to  the  depth,  when  the  same  stra- 
tum is  concerned  and  its  mean  value,  called 
the  geothermic  degree  is  i°  C.  [1.8°  F.]  for  33 
metres  [108.5  ft.]  depth,  that  is  about  3°  C. 
[5.4°  P.]  per  100  metres  [328.5  ft.],  30°  C.  [54° 
P.]  per  kilometre  [.62  mile],  or  3000°  C.  [5400°  P.] 
for  100  kilometres  [62  miles].1  Therefore,  if  the 
terrestrial  crust  was  only  100  kilometres  in  thick- 
ness the  strata  at  that  depth  would  exist  at  a 
temperature  of  3000°  C.  [5400°  P.],  which  is  high 
enough  to  melt  and  vaporise  all  known  bodies. 
We  may  remark  here  that  the  Earth's  shell  does 
not  attain  such  a  thickness  as  this;  the  actual 
thickness  of  the  solid  crust  surrounding  the 
nucleus  is  not  more  than  70  kilometres  [43.5 
miles].  In  proportion  to  its  diameter,  the  ter- 
restrial crust  is  much  thinner  than  the  shell  of 
an  egg. 

Metals,  therefore,  in  a  state  of  fusion  constitute 
the  magma  which  forms  the  central  mass  of  our 
globe.  The  immense  pressures  to  which  they  are 
subjected  must  not  be  forgotten;  these  pressures 

xAt  an  increase,  in  other  words,  of  about  i°  F.  for  every 
60  ft  — Ed. 


Form  and  Mass  of  tHe  EartH         89 

reach  and  surpass  that  of  millions  of  atmospheres. 
It  is  quite  impossible  to  represent  to  the  mind, 
even  with  the  help  of  Amagat's  results,  the  state 
in  which  a  body  exists  when  subjected  at  the 
same  time  to  such  a  high  temperature  and  great 
pressure.  Doubtless  the  mass  is  of  a  consist- 
ency practically  equivalent  to  the  solid  state.  It 
would  only  be  in  the  immediate  neighbourhood  of 
the  solid  crust,  just  below  it,  that  the  outer  layers 
of  the  central  mass,  not  subjected  to  such  great 
pressures  as  those  deeper  down,  could  exist  in  the 
fluid  state,  constituting  the  molten  matter  which 
volcanoes  in  eruption  eject. 

These  liquid  masses  must  exhibit  the  pheno- 
menon of  convection  currents  which  mix  them  to- 
gether and  agitate  them,  and  which  communicate 
their  impulses  to  the  crust  above.  "  How  are  they 
produced,  these  perpetual  movements,  which  are 
a  manifestation  of  the  incessant  "life"  of  the 
Earth?  Is  the  enclosing  crust  itself  stable,  or 
subject  to  continual  vicissitudes?  The  study  of 
luni-solar  action,  of  the  "  tides "  of  the  crust,  and 
of  the  shocks  to  which  it  is  subjected  will  enable 
us  to  answer  these  questions. 


CHAPTER  IV 

THE  MOVEMENTS  OF  THE  EARTH 

A  X  7"E  have  passed  in  imagination  the  birth  and 
*  »  youth  of  the  Earth,  and  the  evolution  of 
its  adolescence.  We  have  also  learned  its  form  and 
dimensions.  It  is  now  time  to  endeavour  to  find 
out  in  what  way  it  lives,  to  speak  figuratively, 
and  in  what  way  it  is  evolving  in  the  actual  pre- 
sent. Possibly  also  with  this  knowledge  we  may 
be  able  to  foresee  what  will  happen  to  it  in  future 
ages. 

The  primary  manifestation  of  life  is  movement, 
so  that  we  will  first  study  the  movements  of  the 
Earth.  These  movements  are  numerous  and 
some  are  very  complex,  but  there  are  two  chief 
ones,  the  movement  of  rotation  and  the  movement 
of  translation. 

The  Earth  turns  on  itself;  it  rotates  round  a 
line,  an  ideal  axis,  which  is  called  the  line  of  the 
poles,  and  which  is  nearly,  but  not  quite,  fixed 
with  regard  to  the  terrestrial  spheroid;  the  poles 

90 


THe  Movements  of  the  Earth        91 

are  the  points  where  this  axis,  if  real,  would  cut 
the  rotating  surface  of  the  geoid  at  a  given  time. 

All  our  measurement  of  time  is  based  on  this 
movement  of  rotation,  which  governs  the  length 
of  the  day.1  y 

The  angular  velocity  of  the  Earth,  which  is  the 
angle  through  which  it  turns  during  a  unit  of 
time,  is  not  very  great.  A  radius  to  the  Earth's 
centre  at  the  equator  sweeps  out  a  sector  of  only 
15°  in  the  course  of  an  hour.  But,  in  spite  of  the 
small  angular  velocity,  the  circumferential  velo- 
cities are  considerable.  Thus  a  point  on  the 
equator  moves  465  metres  [1525  ft.]  every  second 
because  of  the  Earth's  rotation.  This  almost 
equals  the  initial  velocity  of  the  bullets  of  the  old 
infantry  rifle  1874  pattern,  called  the  Gras  rifle. 
At  a  latitude  nearer  the  pole,  the  velocity  decreases 
markedly ;  at  Paris,  it  is,  however,  still  considerable, 
as  will  be  judged  from  the  fact  that  as  I  write 
these  lines  I  am  carried  along  with  a  velocity  of 


1  The  true  solar  day  varies  in  the  course  of  a  year  as  much  as 
31  minutes;  for  this  reason  the  standard  day  is  determined  by  the 
average  time  between  solar  noons,  being  called  the  mean  solar 
day,  which  is  always  24  hours  in  length.  The  true  rotation  time 
of  the  Earth,  however,  is  23  h.  56  m.  4.09  s.  This  is  known  as  the 
sidereal  day.  Because  of  the  Earth's  revolution  about  the  Sun, 
the  former  must  on  the  average  make  slightly  more  than  one 
rotation  to  bring  the  Sun  again  on  the  meridian.  This  extra 
part  of  a  rotation  takes  3  m.  56  s. — Ed. 


92  The  EartK 

365  metres  [1213  ft.]  per  second.  This  equals  the 
initial  velocity  of  the  bullet  of  a  service  pattern 
revolver. 

The  discovery  of  the  Earth's  rotation  forms  a 
noble  page  in  the  history  of  the  conquests  of  the 
human  mind.  Rational  deductions  led  Copernicus 
and  Galileo1  to  this  conclusion,  and  we  are  well 
aware  of  the  storm  which  the  announcement  of 
the  fact  produced.  Little  by  little,  the  discoveries 
made  by  astronomers  added  unforeseen  knowledge 
which  raised  to  the  level  of  absolute  certainty 
what  many  minds  still  wished  to  treat  as  a 
supposition. 

For  example,  Newton's  law  necessitates  that 
a  celestial  body  describing  a  closed  orbit  around 
a  fixed  point  must  be  attracted  by  a  force  acting 
from  that  point.  Such  attraction  implies  a  mass 
concentrated  at  this  point,  that  is  to  say  another 
body,  since  volume  is  inseparable  from  mass. 
Consequently  if  the  stars  rotate  around  the  polar 
axis  of  the  immovable  Earth,  the  line  round  which 
they  appear  to  describe  circular  orbits,  it  follows 


'In  the  5th  century  B.C.,  some  of  the  Greek  nature  philoso- 
phers, notably  Pythagoras,  Democritus,  and  Heraclitus,  evolved 
the  essentials  of  the  theory  revived  by  Copernicus  and  bearing 
his  name.  Their  theories,  however,  were  not  accepted  by  the 
majority  of  their  contemporaries  and  were  later  totally  aban- 
doned for  the  false  Ptolemaic  theory. — Ed, 


THe  Movements  of  tKe  EartH        93 

that  an  attracting  body  must  lie  in  the  centre  of 
each  such  orbit,  one  for  every  star.  There  would, 
therefore,  be  a  succession  of  celestial  bodies,  all 
of  large  mass,  showing  an  alignment  along  the 
Earth's  axis  produced.  Astronomers  have  never 
observed  any  such  alignment,  which  could  not 
have  remained  unnoticed. 

Furthermore,  observation  has  made  it  quite 
certain  that  the  Earth  is  one  of  the  planets  be- 
longing to  the  Solar  System.  It  is  an  established 
fact  that  all  these  planets  rotate  on  their  axes 
and  there  is  no  reason  why  the  Earth  should  be 
the  only  exception  to  the  common  law,  particu- 
larly as  it  is  by  no  means  the  largest  or  most 
massive  one. 

In  the  middle  of  the  nineteenth  century,  Fou- 
cault  furnished  direct  experimental  proofs  of  the 
Earth's  rotation  by  his  two  experiments,  the  great 
pendulum  in  the  Pantheon  at  Paris  and  the 
gyroscope. 

Starting  with  the  knowledge,  capable  of  experi- 
mental proof,  that  a  pendulum  swings  in  a  fixed, 
unchanging  plane  whatever  movement  its  point 
of  support  undergoes,  Foucault  in  1851  attached 
a  wire  70  metres  [229  ft.  ]  long  to  the  keystone  of 
the  dome  of  the  Pantheon.  This  wire  carried  a 
ball  weighing  18  kilograms  [39.75  Ibs.],  below 


94  The  Earth 

which  was  fixed  a  pointed  style;  the  whole  thus 
formed  a  long  pendulum,  oscillating  very  slowly. 
At  the  extremity  of  each  swing,  the  style  of  the 
pendulum  cut  through  a  heap  of  sand  placed  near 
the  farthest  point  it  reached.  In  order  to  start 
the  oscillation  without  introducing  any  extra- 
neous movement,  the  ball  was  pulled  to  one  side 
and  fastened  to  a  fixed  support  by  a  thread  which 
was  subsequently  burnt;  the  pendulum  then 
began  to  swing  freely.  Now,  at  every  oscillation 
the  breach  made  in  the  pile  of  sand  was  observed 
to  widen,  in  the  direction  to  the  left  of  an  observer 
looking  at  it  from  the  far  side.  As  it  was  known 
that  the  plane  of  swing  was  invariable,  and  con- 
stituted a  fixed  reference  direction,  the  experiment 
showed  that  the  support  was  being  displaced. 
In  other  words,  the  Pantheon,  and  consequently 
the  whole  Earth  itself,  was  rotating  in  a  contrary 
way  to  that  in  which  the  breach  was  widened. x 

Foucault  wished  to  do  still  more,  and  he  suc- 
ceeded. He  invented  the  gyroscope.  Imagine  a 
series  of  pendulums  side  by  side  all  of  equal  length, 
all  carrying  identical  masses  and  swinging  about 

1  In  1902  on  the  occasion  of  the  5Oth  anniversary  of  this  de- 
monstration, Foucault's  experiment  was  formally  repeated  in 
the  Pantheon,  in  the  presence  of  the  chief  State  officials,  under 
the  same  conditions  as  when  originally  made.  The  author  of 
this  work  had  charge  of  the  experiment. 


THe  Movements  of  tHe  EartH        95 

the  same  axis,  but,  instead  of  describing  only  the 
arc  of  the  circle,  let  them  be  supposed  to  complete 
the  entire  circle,  all  swinging  in  the  same  plane, 
perpendicular  to  the  common  axis  of  rotation.  A 
gyroscope,  which  is  actually  a  heavy  flywheel  of 
small  radius  turning  very  rapidly  around  its  axis, 
would  act  similarly  to  such  a  series,  and  so  should 
maintain  an  invariable  direction.  If  one  of  the 
points  of  the  Cardan  suspension  of  this  instru- 
ment be  observed  by  means  of  a  view-telescope 
it  will  appear  to  move  in  the  opposite  way  to 
the  Earth's  rotation,  which  latter  is  thus  once 
more  demonstrated. 

The  applications  of  gyroscopic  action  that  have 
already  been  made  are  well  known.  Sailors  have 
utilised  it  to  give  a  fixed  horizon  when  a  distant 
fog  hides  the  sea  horizon,  it  being  unaffected  by 
the  movement  of  the  ship.  It  is  proposed  to  make 
use  of  it,  also,  instead  of  the  magnetic  needle  to 
give  the  true  north,  and  aviators  have  attempted 
to  utilise  it  for  rendering  their  machines  more 
stable.1 

When  Foucault's  experiment  is  attentively 
observed,  not  only  qualitatively,  but  also  quanti- 


1  As  this  English  translation  goes  to  press  the  newspapers 
are  reporting  a  successful  gyroscopic  stabiliser  for  aeroplanes. — 
Ed. 


96  The  Earth 

tatively,  if  the  expression  may  be  used,  one  is 
struck  by  a  fact  which,  at  first  sight,  seems 
inexplicable  and  contradictory  but  which  has 
really  quite  a  simple  explanation. 

If  the  angle  through  which  the  plane  of  oscil- 
lation of  the  pendulum  seems  to  turn,  for  example, 
during  one  hour,  be  measured,  it  is  found  that  its 
value  corresponds  at  Paris  to  a  velocity  of  rota- 
tion of  one  complete  turn  in  36  hours.  Now,  it 
is  quite  certain  that  the  Earth's  angular  velocity 
is  one  rotation  in  24  hours  [23  h.  56  m.  4  s.],  since 
it  is  this  velocity  which  gives  us  the  definition  of  the 
unit  of  time,  a  day,  and  its  subdivision,  an  hour. 
There  is  thus  an  apparent  paradox.  The  reason 
is  that  the  pendulum  experiment  shows  the  rota- 
tion, not  around  the  line  of  the  poles  but  around 
the  vertical  line  at  the  place  of  observation,  in 
this  case  the  vertical  to  the  surface  at  Paris.  In 
order  to  find  the  velocity  of  rotation  with  reference 
to  this  vertical  we  must  multiply  the  actual 
velocity  by  the  sine  of  the  angle  of  latitude  of  the 
place  of  experiment.  If  this  calculation  be  made 
for  the  latitude  of  Paris  it  is  found  that  the  appar- 
ent velocity  of  rotation  of  the  plane  of  swing  of 
the  pendulum  or  of  the  gyroscope  is  exactly  one 
turn  in  thirty-six  hours. 

As  a  consequence  of  this,  if  the  experiment  be 


The  Movements  of  the  EartK        97 

made  in  gradually  increasing  latitudes,  that  is 
going  northwards,  the  velocity  of  rotation  will 
increase,  and  at  the  pole  itself,  where  the  Earth's 
axis  and  the  vertical  at  the  place  coincide,  the 
pendulum  would  appear  to  describe  one  turn  in 
twenty-four  hours  [23  h.  56  m.  4  s. — Ed.].  Arctic 
or  Antarctic  explorers  who  have  the  good  fortune 
to  reach  the  actual  pole,  and  who  can  stay  there 
long  enough  to  study  such  phenomena,  would  find 
this  a  most  interesting  experiment  to  try.  In  pro- 
portion as  the  equator  is  approached,  on  the 
other  hand,  the  velocity  with  which  the  pendulum 
appears  to  turn  becomes  smaller  and  smaller 
until  at  the  equator  itself,  the  pendulum  oscillates 
in  a  constant  direction.  This  has  been  tested  by 
experiment.  In  the  southern  hemisphere  the 
phenomena  are  identical,  but  the  apparent  motion 
is  in  the  opposite  direction. 

The  scientific  value  of  Foucault's  experiment 
is  considerable.  Thus  if  the  apparent  velocity 
of  rotation  of  a  gyroscopic  apparatus  be  measured 
with  care,  the  latitude  of  the  place  can  be  deduced. 
Now  the  latitude,  defined  astronomically,  has  not 
been  hitherto  obtainable  save  by  the  observation 
of  stars,  that  is  to  say,  by  means  of  celestial  refer- 
ence points.  Foucault's  method  requires  neither 
a  view  of  the  sky  nor  observation  of  stars  and,  by 

7 


98  The  Earth 

using  it,  an  astronomically  defined  co-ordinate  may 
be  determined  at  the  bottom  of  a  vault  or  other 
place  from  which  no  part  of  the  heavens  is  visible. 

If  it  should  become  possible  to  arrange  an  appa- 
ratus enabling  this  experiment  to  be  made  with 
sufficient  precision  on  board  a  ship  in  motion,  an 
important  problem  will  have  been  solved,  for 
sailors  would  thus  be  able  to  take  their  reckoning 
even  when  the  Sun  or  stars  were  hidden  by  fog 
or  clouds. 

The  practical  consequences  of  the  Earth's  rota- 
tion are  of  supreme  importance,  and  it  may  be 
said  that  the  essential  features  of  the  system  of 
atmospheric  circulation,  and  also,  in  part,  those 
of  the  oceanic  circulation,  result  therefrom. 

The  science  of  mechanics  shows  that,  as  a  con- 
sequence of  the  theorem  of  Coriolis,  any  moving 
body  on  a  sphere  rotating  upon  its  axis,  as  the 
Earth  does,  is  by  reason  of  this  rotation  deviated 
in  a  direction  to  the  right  of  its  path  in  the  northern 
hemisphere  and  to  the  left  in  the  southern  hemi- 
sphere. Accordingly  when  we  have  to  study  the 
great  movements  of  translation,  whether  of  the 
gaseous  masses  which  constitute  the  atmosphere, 
or  of  the  liquid  masses  forming  the  oceans,  it  will 
be  necessary  to  take  account  of  the  permanent 
action  of  that  deviating  force. 


THe  Movements  of  tKe  Earth.        99 

This  deviating  force  is,  nevertheless,  very  feeble. 
We  know  by  mechanics  that  it  is  proportional  to 
the  angular  velocity  of  the  rotating  sphere,  to  the 
mass  of  the  body  moving  on  the  surface  of  the 
sphere,  to  the  linear  velocity  of  this  body,  and 
also  to  the  sine  of  the  latitude.  For  a  body  of 
mass  one  gram  [15432  gr.]  moving  with  a  velocity 
of  one  metre  [39.37  in.]  per  second  at  the  latitude 
of  45°  it  would  be  about  iiroTooro  part  of  the 
weight  of  the  body  (to  be  exact  9 8.000  part). 
But  this  force  acts  on  considerable  masses,  moving 
with  large  velocities  over  long  distances,  and, 
therefore,  it  is  not  surprising  that  it  exerts  a 
marked  action  upon  the  paths  of  the  fluid  masses 
which  circulate  around  the  solid  crust  of  the  Earth. 

Another  consequence  of  the  deviating  force 
exerted  upon  moving  bodies  is  shown  in  the  east- 
ward deviation  suffered  by  heavy  masses  falling 
freely  towards  the  Earth's  surface.  This  devia- 
tion is  caused  by  the  fact  that  the  linear  velocity 
of  the  elevated  point  from  which  the  body  com- 
mences to  fall  is  greater  than  that  of  the  point 
where  it  strikes  the  Earth,  since  the  former  is 
farther  from,  and  the  latter  nearer  to,  the  axis 
of  rotation.  The  body  therefore  falls  at  a  point 
advanced  towards  the  east. 

This  deviation  may  be  calculated  by  the  prin- 


loo  TKe  Earth 

ciples  of  mechanics;  it  is  very  slight.  If  the  cal- 
culation be  made  for  the  equator  where  the  linear 
velocities  of  rotation  are  greatest,  it  is  found  that 
a  body  falling  from  a  height  of  100  metres  [326.5 
ft.]  will  be  deviated  33  millimetres  [3.93  in.]  to- 
wards the  east.  The  extreme  smallness  of  this 
deviation  has  always  hindered  the  conclusive 
proof  of  the  phenomenon,  the  existence  and 
magnitude  of  which  are  however  indisputable. 

We  must  now  consider  another  result  of  the 
Earth's  rotation.  The  movement  brings  into 
play  a  centrifugal  force  which  is  greater  in  propor- 
tion as  the  point  of  the  surface  in  question  is 
farther  from  the  axis.  At  the  equator,  therefore, 
this  force  attains  its  maximum ;  it  tends  to  neutral- 
ise the  force  of  gravity,  which  it  actually  diminishes 
by  TS¥  part  of  the  total  value.  In  other  words,  at 
the  equator,  the  centrifugal  force  is  -^ww  of  the 
force  of  gravity. 

As  the  centrifugal  force  is  proportional  to  the 
square  of  the  angular  velocity  of  the  rotating 
body,  and  since  289  is  the  square  of  17,  it  will 
be  seen  that,  if  the  Earth  were  to  turn  17  times 
more  rapidly  than  at  present,  the  force  of  gravity 
would  at  the  equator  be  exactly  counterbalanced 
by  the  centrifugal  force,  and  hence  nothing  there 
would  have  any  apparent  weight. 


The  Movements  of  the  EartH      itbi 

The  consequence  of  such  a  state  of  affairs  would 
be  disastrous  as  regards  the  living  beings  on  the 
Earth.  In  the  first  place,  our  bodies  would  weigh 
nothing  and  exercise  no  pressure  on  the  ground, 
and  hence  no  friction ;  walking  and  running  would 
be  rendered  impossible,  as  also  would  be  the 
movement  of  railway  trains  over  their  rails  which 
they  would  not  grip.  A  jump,  the  result  of  a 
spring  given  by  muscular  effort,  would  carry  a 
person  to  an  enormous  height,  which  would  only 
be  limited  by  the  resistance  of  air,  if  the  air  could 
exist  in  the  circumstances,  but  probably  neither 
air  nor  water  could  remain  at  the  Earth's  surface. 
As  regards  liquids,  they  would  no  longer  collect 
in  the  bottom  of  their  receptacles;  the  oceans, 
driven  by  the  winds,  would  accumulate  on  their 
shores  forming  mountains  of  water,  which  would 
not  tend  to  fall  back  again  and  become  horizontal 
sheets.  It  would  not  be  possible  to  pour  wine  or 
water  into  glasses  and  they  would  not  even  flow 
out  of  their  bottles. 

All  manual  trades  would  be  rendered  impossible 
as  their  fundamental  instrument,  the  hammer, 
would  not  do  its  work,  on  account  of  the  disap- 
pearance of  its  weight.  No  body  would  fall ;  there 
would  thus  be  no  indication  of  the  vertical  direc- 
tion by  a  plumb  line  or  of  the  horizontal  by  a  level. 


102  The  Earth 

Similar  remarks  could  be  multiplied  to  show 
how  impossible  existence  would  be  under  these 
fortunately  only  imaginary  conditions. 

Independently  of  its  movement  of  rotation 
around  the  polar  axis,  the  Earth  has  a  movement 
of  translation,  viz.,  its  revolution  around  the  Sun. 
This  motion  takes  place  in  conformity  with  Kep- 
ler's laws,  that  is  to  say  the  Earth's  centre1  describes 
an  ellipse,  one  focus  of  which  is  occupied  by  the 
Sun.  Our  planet  does  not  move  in  its  orbit  with 
uniform  velocity;  the  second  law  of  Kepler,  which 
deals  with  the  areas  swept  out  during  equal  periods 
of  time  states  that  the  Earth  moves  quickest  in 
its  orbit  when  at  its  nearest  point  to  the  Sun  and 
slowest  when  at  the  farthest  point. 

The  major  axis  of  this  elliptical  path,  the  pro- 
jection of  which  on  the  celestial  sphere  is  called 
the  ecliptic,  has  a  length  of  297,500,000  kilometres 
[185,450,000  miles]  and  the  eccentricity  of  the 
ellipse  is  about  £0.  The  mean  distance  of  the 
Earth  from  the  Sun  is  therefore  148,000,000  kilo- 
metres [93,000,000  miles],  which  is  rather  more 
than  23,000  times  the  radius  of  the  Earth. 

The  orbit  is  traversed  by  the  centre  of  our  globe 
in  one  year  and  the  total  length  of  this  yearly  path 

1  Strictly  speaking  it  is  the  centre  of  gravity  of  the  Earth- 
Moon  system  which  describes  this  elliptical  orbit. — Ed. 


The  Movements  of  tKe  Earth      103 

equals  930,000,900  kilometres  [577,000,000  miles]. 
This  corresponds  to  a  mean  velocity  of  106,000 
kilometres  [66,000  miles]  per  hour,  a  speed  far 
beyond  the  dreams  of  even  the  most  ambitious 
of  our  motorists  or  aviators. 

But  the  Earth  does  not  always  travel  with  this 
mean  velocity ;  in  accordance  with  Kepler's  second 
law  it  moves  sometimes  more  quickly  and  some- 
times less  quickly.  It  will  be  interesting  to  give 
the  extreme  velocities  and  to  express  them  not  in 
kilometres  per  hour  but  in  metres  per  second. 
At  the  period  of  the  summer  solstice,  when  the 
Earth  is  farthest  from  the  Sun  and  consequently 
moves  most  slowly,  it  passes  over  28,900  metres 
[17.9  miles]  per  second,  while  about  January  1st 
its  velocity  is  30,000  [18.6  miles]  metres  in  the 
same  interval  of  time.  No  projectile  attains  a 
speed  comparable  to  this;  the  bullets  of  the  most 
rapid  rifles,  using  a  powder  such  as  cordite, 
scarcely  attain  a  speed  of  a  thousand  metres 
[3,200  ft.]  per  second.  This  is  very  different  from 
the  Earth's  velocity  of  translation,  with  which  the 
reader  is  carried  while  he  reads  this  page,  a  ve- 
locity which  he  cannot  suspect  because  of  the 
total  lack  of  any  points  or  objects  near  the  Earth 
by  which  to  gauge  the  movement. 

We  shall  now  consider  how  the  two  movements, 


104  The  Earth 

rotation  and  revolution,  are  combined.  The  first 
illustration  that  may  be  given  is  that  of  a  rolling 
ball,  moving  forward  around  an  elliptical  railway; 
while  traversing  the  ellipse  it  also  rotates  upon 
itself,  and,  during  one  rotation,  it  progresses  over 
the  rails  a  distance  equal  to  the  circumference  of 
one  of  its  great  circles.1 

But,  in  order  for  this  simple  combination  of  the 
movements  to  take  place,  there  must  be  a  suitable 
relation  between  the  velocity  of  rotation  and  that 
of  revolution.  In  the  case  of  the  Earth,  the  velo- 
city of  rotation  is  only  ^  part  of  that  of  revolu- 
tion. If  we  wish  to  represent  the  Earth's  double 
movement  in  one  place  by  that  of  a  ball,  we  must, 
therefore,  imagine  that  the  ball  not  only  rolls,  but 
also  slides  at  the  same  time,  in  such  a  way  as  to 
turn  on  its  axis  only  once  in  twenty-four  hours. 

The  game  of  billiards  naturally  suggests  the 
idea  that  it  might  be  possible  to  demonstrate  the 
Earth's  movements  on  a  table  assumed  to  be 
frictionless. 

If  we  admit  that  the  double  movement  of  the 
Earth  is  the  result  of  the  impulse  of  some  cosmic 
force  (which  is  a  quite  gratuitous  hypothesis,  a 
purely  mental  speculation)  it  can  easily  be  calcu- 

1 A  great  circle  is  the  maximum  circumference  of  a  sphere. — 
Trans. 


THe  Movements  of  tKe  EartH      105 

lated  that  such  a  force  could  not  have  been  directed 
towards  the  Earth's  centre,  but  towards  a  point 
situated  to  one  side  of  it,  on  the  radius  perpendi- 
cular to  the  direction  of  the  force,  distant  about 
33,750  metres  [20  miles]  from  the  centre,  which 
quantity  is  TST  of  the  radius. 

If,  therefore,  we  wish  to  represent  the  Earth's 
motion  on  a  billiard  table,  we  must  take  a  ball 
of  189  millimetres  [7.37  in.]  radius,  that  is  to 
say,  37.8  centimetres  [14.7  in.]  in  diameter.  The 
material  of  which  the  ball  is  made  should  be  formed 
in  concentric  layers  of  the  same  densities  as,  and 
proportional  in  thickness  to,  the  corresponding 
ones  of  our  globe.  Then  we  must  take  a  perfectly 
straight  cue  and  hit  the  ball  eccentrically  in  such 
a  way  that,  at  the  moment  of  striking,  the  direction 
of  the  cue  passes  through  a  point  a  millimetre 
[•°3937  m-]  from  the  centre.  The  ball  will  then 
have  a  combined  movement  of  rotation  and  revo- 
lution, the  velocities  of  which  component  move- 
ments will  be  in  the  same  ratio  as  the  actual 
corresponding  ones  of  the  Earth. 

Finally,  it  may  be  remarked  that,  during  the  kind 
of  waltzing  movement  that  our  planet  performs 
around  the  Sun,  the  polar  axis  is  not  perpendicular 
to  the  plane  of  the  orbit.  It  is  not  upright,  but 
inclined  to  the  ecliptic,  in  such  a  way  that  the 


106  The  Earth 

plane  of  the  terrestrial  equator  and  that  of  the 
ecliptic  make  an  angle  of  about  23  3°  with  one 
another.  The  polar  axis,  therefore,  makes  the 
angle  complementary  to  this,  viz.,  66^°,  with  the 
plane  of  the  Earth's  orbit.  This  axial  inclination 
is  of  primary  importance  in  connection  with  the 
life  existing  on  the  Earth;  it  is  the  reason  of  the 
inequality  of  the  days  and  nights,  and  it  is  also 
the  cause  of  the  seasons  which  succeed  one  another 
during  the  course  of  a  year. 

If  the  Earth  were  absolutely  spherical,  and  not 
accompanied  by  its  satellite,  the  Moon,  the  move- 
ments above  described  would  be  the  only  ones 
that  our  planet  would  experience.  But  the  Earth 
is  not  spherical,  and  in  addition  to  this,  it  has  a 
satellite  revolving  round  it.  Consequently  other 
movements  are  also  imposed  upon  the  Earth. 
The  non-sphericity  takes,  as  we  have  said,  the 
form  of  a  polar  flattening  and  an  equatorial  bulge. 
The  effect  of  the  latter  is  very  important.  When 
it  is  desired  to  find  the  attraction  exerted  by  a 
given  mass  on  a  body,  one  can  make  the  calcula- 
tion as  if  all  the  mass  of  the  body  was  accumulated 
at  its  centre,  provided  it  be  spherical  and  homo- 
geneous. The  problem  is  thus  simplified  and  the 
result  absolutely  accurate.  But  the  equatorial 
bulge  renders  the  Earth  not  truly  spherical,  and, 


TKe  Movements  of  tHe  EartK      107 

therefore,  the  attractive  forces  exerted  on  it  by 
neighbouring  celestial  bodies  are  unsymmetrical 
excepting  in  the  case  where  such  bodies  are  situated 
on  the  line  of  the  Earth's  poles  or  in  the  plane  of 
its  equator.  Save  in  these  two  special  cases,  a 
neighbouring  body,  such  as  the  Moon  or  the  Sun, 
will  be  at  differing  distances  from  the  two  halves 
of  the  equatorial  bulge  and  therefore  attract  them 
in  different  degrees.  Consequently  the  effect 
will  be  a  tendency  to  turn  the  Earth  over. 

The  Moon  and  the  Sun  both  exert  an  appreci- 
able effect  of  this  kind.  In  spite  of  the  small  mass 
of  the  former,  only  ^th  part  of  that  of  the  Earth, 
it  produces  the  larger  effect  on  account  of  its  prox- 
imity, its  distance  from  the  centre  of  the  Earth 
being  only  thirty  times  the  Earth's  diameter. 
On  the  other  hand  the  Sun  is  a  very  great  distance 
away,  viz.,  11,500  terrestrial  diameters,  and  in 
accordance  with  the  law  of  gravitation  the  attrac- 
tive force  is  inversely  as  the  square  of  this  distance. 
This  is  partially  compensated  for  by  the  enormous 
mass  of  the  Sun,  which  is  324,000  times  greater 
than  that  of  our  planet. 

Thus  the  Sun,  in  virtue  of  its  great  mass,  and 
the  Moon  by  reason  of  its  nearness,  continuously 
act  together  to  change  the  direction  of  the  axis 
around  which  the  Earth's  rotation  takes  place. 


108  The  Earth 

The  line  of  the  poles  makes  an  angle  of  23^°  with 
the  perpendicular  to  the  plane  of  the  ecliptic; 
under  the  influence  of  the  luni-solar  attraction, 
the  former  performs  a  slow  rotatory  movement, 
from  east  to  west,  in  such  a  way  as  to  describe 
a  cone,  the  apex  of  which  is  at  the  Earth's  centre. 
The  cone  is  described  once  in  about  26,000  years, 
after  which  the  terrestrial  axis  repeats  the  same 
successive  positions  in  the  next  period  of  26,000 
years  and  so  on.  Thus  the  Earth  while  turning 
rapidly  around  its  axis  executes  a  supplementary 
movement,  resulting  in  the  conical  displacement 
of  that  axis;  nothing  could  better  illustrate  this 
than  a  top  which  while  it  spins  quickly  about  its 
axis  slowly  describes  a  cone  whose  apex  is  at  its 
point. 

The  precession  of  the  equinoxes  is  the  name 
given  to  the  movement  executed  by  the  Earth's 
axis  during  every  26,000  years.  It  has  important 
results,  from  the  point  of  view  of  the  life  on  the 
globe.  In  the  course  of  ages  precession  changes 
the  relative  duration  of  the  seasons,  and  thus 
produces  secular  variations  of  the  general  climatic 
conditions. * 

The  cone-shaped  figure  described  by  the  line 

1  A  secular  variation  is  a  progressive  or  permanent  change 
from  the  average  condition. — Trans. 


TKe  Movements  of  tHe  Earth.      109 

of  the  poles  is  not  in  itself  quite  regular;  its  edge 
is  not  truly  circular  but  serrated  or  wavy,  some- 
what like  the  cardboard  in  which  fragile  objects 
are  packed.  The  explanation  of  this  fact  is  as 
follows:  Astronomers  teach  us  that  the  plane  of 
the  lunar  orbit  suffers  a  periodical  displacement, 
and  that  its  point  of  intersection  with  the  plane 
of  the  Earth's  orbit  moves  from  east  to  west, 
performing  a  complete  revolution  in  i8£  years. 
During  this  period  the  Earth's  polar  axis  is  slightly 
displaced,  sometimes  interior  to  and  sometimes 
exterior  to,  the  theoretical  conical  surface  that  it 
should  describe  in  accordance,  with  the  phenome- 
non of  precession.  As  a  consequence,  the  projec- 
tion of  the  axis  on  the  celestial  sphere  describes 
in  i8J  years  a  little  ellipse  whose  axes  have  the 
angular  measurement  of  36  and  18  seconds  of 
arc  respectively.  This  phenomenon  is  called 
nutation. 

This  does  not  exhaust  the  list  of  the  Earth's 
motions.  In  its  apparent  movement  round  the 
Earth,  the  Sun  every  six  months  seems  to  cross  from 
one  side  of  the  equator  to  the  other  as  it  passes 
through  the  equinoxes.  In  the  precise  language  of 
astronomers  and  sailors,  its  declination  changes 
sign.  In  consequence  of  the  equatorial  pro- 
tuberance, the  part  of  which  turned  towards  the 


no  The  Earth 

Sun  is  more  strongly  attracted  than  the  other  part, 
the  projection  of  the  terrestrial  axis  on  the  celestial 
sphere  oscillates  around  a  small  ellipse  whose 
centre  is  always  situated  on  the  undulated  curve 
arising  from  precession  and  nutation,  and  whose 
angular  dimensions  are  respectively  2  seconds  and 
I  second  of  arc.  Thus  the  serrations  due  to  nuta- 
tion are  themselves  serrated  by  this  phenomenon, 
the  period  of  which  is  six  months. 

Furthermore  the  Moon  passes  every  fourteen 
days  from  one  side  of  the  equator  to  the  other  and 
this  produces  a  fourth  oscillation  of  the  terrestrial 
axis  which  describes  a  fourth  very  small  ellipse, 
the  centre  of  which  remains  on  the  circumference 
of  the  preceding  one,  but  whose  angular  dimen- 
sions are  only  four-tenths  and  two-tenths  of  a 
second  of  arc  respectively. 

If  we  therefore  try  to  summarise  all  these  per- 
turbations affecting  the  rotatory  movement  of 
the  Earth,  we  arrive  at  the  following  law:  If  the 
terrestrial  polar  axis  be  produced  until  it  meets 
the  imaginary  spherical  surface  representing  the 
heavens,  a  surface  which  is  frequently  made  use 
of  in  astronomical  reasoning  and  which  is  called 
the  celestial  sphere,  the  line  so  prolonged  does  not 
meet  it  in  a  fixed  point,  even  when  the  revolution 
of  the  Earth  round  the  Sun,  and  its  movement  in 


The  Movements  of  tHe  EartH      in 

space,  are  not  taken  into  account.  Every  four- 
teen days  it  describes  a  very  small  ellipse,  the 
centre  of  which  moves  in  six  months  around  a 
second  and  rather  smaller  ellipse,  caused  by  the 
displacement  of  the  Sun  in  declination.  The 
centre  of  this  latter  ellipse  moves  in  a  third  and 
much  larger  ellipse,  every  i8J  years,  viz.,  that  of 
nutation,  while  finally  the  centre  of  this  large  el- 
lipse in  26,000  years  traverses  the  circumference  of 
the  circle  due  to  the  precession  of  the  equinoxes. 
The  curve  that  truly  represents  the  Earth's  move- 
ment of  rotation  is  therefore  one  with  four  com- 
bined sets  of  serrations  or  indentations. 

We  may  now  put  to  ourselves  a  new  question. 
Does  the  Earth's  polar  axis,  although  describing 
a  curve  of  great  complexity  on  the  celestial  sphere, 
remain  absolutely  fixed  with  reference  to  the  ter- 
restrial crust?  In  other  words,  does  the  actual 
pole  of  the  Earth  remain  fixed  relatively  to  the 
surface?  The  answer  is  in  the  negative.  The 
pole  is  not  so  fixed;  it  moves  slowly  over  the  solid 
crust,  the  range  of  movement  being  however  very 
small,  though  continual.  If  the  Earth  be  repre- 
sented by  a  wooden  ball  turning  upon  an  axis  of 
steel,  it  is  as  if  this  axis  shook  about  slightly, 
instead  of  being  firmly  fixed  inside  the  ball  which 
turns  on  it.  This  phenomenon  has  been  studied 


112  The  Earth 

by  geodesists  and  astronomers  under  the  name  of 
the  fluctuation  of  latitude. 

How  have  they  been  enabled  to  recognise  such 
a  minute  change?  The  measurements  show,  in 
fact,  that  the  displacement  of  the  pole  on  the 
Earth's  surface  is  comprised  within  the  limit  of  a 
few  metres  [or  yards]  around  the  theoretical  place 
of  the  pole.  The  discovery  and  measurement  of 
this  phenomenon  perhaps  form  the  most  wonderful 
result  of  recent  astro-geodesy.  It  would  appear 
that  no  phenomenon  can  escape  the  eye  of  the 
really  good  observer,  and  this  discovery  again 
shows  the  depth  of  understanding  contained  in 
Pasteur's  thought:  "The  faculty  of  opportune 
speculation  is  the  first  step  along  the  way  of 
discovery." 

The  movement  of  the  poles  was  discovered  as 
follows:  Every  point  of  the  Earth  is  marked  on 
maps  or  on  terrestrial  globes  by  two  quantities: 
(i)  the  longitude,  which  indicates  its  distance  from 
a  fixed  meridian  (that  of  Greenwich  is  the  stand- 
ard for  the  whole  world) ;  (2)  the  latitude,  which 
gives  the  distance  from  the  equator,  reckoned 
along  the  meridian  of  the  point  in  question  (Fig.  9). 
Astronomical  observations  do  not  give  this  latter 
angle  directly,  but  its  complement,  the  colatitude, 
which  is  the  difference  between  it  and  a  right 


THe  Movements  of  tHe  EartH      113 


ZENITH 


angle.     The    problem    of    finding    the    latitude, 
therefore,    resolves    itself    into    determining    the 
angle  made  by  the  Earth's  polar  axis  with  the 
vertical  at  the  place  of 
observation.     Latitude 
determination     is     the 
everyday  occupation    of 
travellers,  sailors,  and 
astronomers ;     terrestrial 
explorers    use    the    the- 
odolite,  sailors  the   sex- 
tant, and  both  easily  give 
results  accurate  to  with- 


FIG.  9. — Latitude  and  Co- 
latitude. 


in  twenty  seconds,  that  is  to  say  the  error  of  position 
would  not  surpass  600  metres  in  the  north-south 
direction.  Astronomers,  with  the  aid  of  meridian 
circles,  can  find  the  latitude  within  less  than  one- 
tenth  of  a  second,  which  means  that  the  possible 
error  of  the  latitude  assigned  to  the  place  of  ob- 
servation does  not  exceed  three  metres ! 

Now,  in  1889  and  1890,  the  observatories  of 
Berlin,  Potsdam,  and  Prague  found  that  their 
latitudes,  frequently  measured  by  the  astronomers 
doing  meridian  work,  varied  continuously,  and 
what  was  even  more  remarkable,  all  three  latitudes 
varied  in  the  same  sense,  as  if  the  North  Pole  was 
slightly  approaching  these  towns.  The  precision 


The  Earth 


of  the  instruments  employed  and  the  experience 
and  ability  of  the  observers  obviated  the  possibil- 
ity of  these  discrepancies  being  merely  fortuitous 
errors,  especially  as  they  were  always  of  the  same 
order  of  magnitude,  viz.,  several  tenths  of  a  second. 
In  face  of  these  facts,  the  International  Geodetic 
Association  determine  to  elucidate  the  matter  as 
completely  as  possible  by  making  a  crucial  ex- 
periment. 

Consider  two  points  A  and  B  of  the  Earth  (Fig. 
10)  at  the  same  distance  from  the  pole,  that  is  to 

say  on  the  same  paral- 
lel, but  opposed  to  one 
another,  their  longi- 
tudes differing  by  180°. 
If  the  North  Pole  moves 
and  thus  becomes  nearer 
to  one  of  them,  passing 
from  P  to  P,  say,  it  will 
recede  an  equal  dis- 
tance from  the  other  point.  Therefore,  if  the 
latitude  of  A  increase  by  a  certain  angle,  that 
of  the  point  B  must  decrease  by  the  same  angle, 
and  vice  versa.  In  1891,  the  International  Geo- 
detic Association  sent  to  Honolulu  the  German 
astronomer  Marcuse  and  the  American  astrono- 
mer Preston.  Honolulu  and  Berlin  occupy  in  re- 


B  A - 


FIG.  10. — Fluctuation  of 
Latitudes. 


THe  Movements  of  tHe  EartH      115 

spect  to  one  another  very  nearly  the  positions  of 
the  points  A  and  B  of  the  figure.  The  result 
was  decisive ;  while  the  latitude  increased  at  Hono- 
lulu, it  decreased  an  equal  amount  at  Berlin.  In 
order  to  attain  absolute  certainty,  six  equidis- 
tant stations  around  the  North  Pole  and  two  others 
near  the  South  Pole  were  established  in  1895.  It 
was  proved  that  at  each  pair  of  opposite  stations, 
the  variations  were  equal  and  of  opposite  sign. 
The  terrestrial  poles  are  therefore  not  fixed  on  the 
Earth's  surface  but  move  without  cessation. 

Having  demonstrated  qualitatively  this  curious 
phenomenon,  it  remained  to  observe  it  quantita- 
tively, that  is,  to  measure  it  with  regard  both  to 
the  time  taken  and  space  traversed. 

In  the  first  place,  a  periodicity  has  been  proved, 
which,  at  first  sight,  seems  to  have  no  relation  to 
the  periods  of  the  Earth's  movements;  the  pole 
returns  to  the  same  meridian  once  in  every  430 
days,  that  is  in  about  fourteen  months.  As  to 
the  spatial  magnitude  of  the  phenomenon,  the 
extent  of  the  motion  reaches  two-  to  three-tenths 
of  a  second  of  arc,  or  expressed  as  distance,  six  to 
ten  metres.  Figure  II  shows  the  journeyings  of 
the  North  Pole  over  the  Earth's  surface,  between 
the  years  1900  and  1910.  It  is  really  remarkable 
that  such  an  intangible  phenomenon  has  been 


The  Earth 


discovered  and  measured.  It  has  only  been 
achieved  by  the  scientific  co-operation  of  all  the 
civilised  nations,  and  such  co-operation  is  becoming 


FIG.  ii.— Wanderings  of  the  North  Pole  (each  side  of  the 
square  equals  about  65  ft). 

more  and  more  the  keynote  of  modern  science. 
Even  in  science,  or  perhaps  it  might  be  said,  espe- 
cially in  science,  does  union  make  for  strength  and 
strength  for  action. 

The  cause  of  the  continuous  polar  motion  is 
still  rather  obscure.  Chandler  in  his  masterly 
papers  on  this  subject  has  attacked  the  problem 


THe  Movements  of  tHe  EartH      117 

from  a  purely  theoretical  point  of  view,  and  has 
given  a  mathematical  analysis  of  it  which  has 
thrown  much  light  upon  the  subject.  By  group- 
ing together  all  the  observations  made  up  to  the 
year  1893,  he  deduced  the  important  result  that 
the  movement  of  the  pole  could  be  expressed  by 
a  formula  containing  two  terms;  the  amplitudes 
of  these  terms,  amplitudes  which  give  the  intensity 
of  the  phenomenon,  vary,  in  the  case  of  the  first 
term  from  r§fo  to  iWo  of  a  second  of  arc,  which 
corresponds  to  a  displacement  on  the  Earth's  sur- 
face of  from  2.64  to  5.70  metres,  and  for  the  second 
term  from  ro¥o  to  iWo  of  a  second,  corresponding 
to  a  distance  of  from  3.56  to  4.30  metres. 

The  periods  of  these  two  terms  are  on  the  aver- 
age, 430  days  for  the  first  and  365  days  for 
the  second,  that  is  fourteen  and  twelve  months 
respectively. 

It  will  at  once  be  seen  how  important  this  re- 
search is.  The  phenomenon  must  be  produced 
by  the  combined  effort  of  two  periodical  actions, 
the  period  of  one  being  fourteen  months,  that  of 
the  other  being  annual.  We  have  therefore  to 
make  a  separate  study  of  the  two  actions. 

As  regards  the  former  there  is  an  astronomical 
cause.  The  periodicity  of  430  days  can  be  ex- 
plained by  an  astronomical  residual,  arising  from 


Ii8  The  Earth 

the  action  of  the  Moon  on  the  equatorial  bulge  of 
the  Earth. 

Lord  Kelvin  and  Newcomb  have  shown  that, 
taken  as  a  whole,  the  terrestrial  globe  has  an 
elasticity  comparable  to  that  of  steel;  we  shall 
find  an  interesting  confirmation  of  this  in  studying 
seismic  phenomena.  Newcomb  has  shown  that 
if  this  be  so  the  period  of  the  polar  displacement 
should  be  427  days,  a  figure  which  agrees  remark- 
ably well  with  the  430  days  indicated  by  Chandler. 
Furthermore,  long-continued  observations  with 
tide  gauges  have  demonstrated  the  fact  of  the 
periodical  rising  and  falling  of  certain  shores,  for 
example  those  of  the  North  Sea  near  Helder  and 
those  of  the  Pacific  Ocean  in  the  neighbourhood 
of  San  Francisco.  The  period  of  these  variations 
is  precisely  fourteen  months  and  is  therefore  equal 
to  that  of  the  first  term  in  Chandler's  formula. 

The  work  of  Professor  Volterra  has  established 
that  every  anomaly  presented  by  the  free  rotation 
of  a  body  can  be  explained  by  movements  which 
would  not  change  either  the  body's  form,  or  the 
intensity  of  the  attraction  it  exercises  on  bodies 
outside  it.  Now,  we  know  that  towards  the 
Earth's  centre  the  excessive  pressure  to  which  the 
fused  materials  constituting  its  interior  is  sub- 
jected gives  to  them  a  rigidity  practically  equi- 


THe  Movements  of  tHe  EartH       119 

valent  to  that  of  the  solid  state ;  on  the  other  hand 
in  the  neighbourhood  of  the  crust,  where  the 
pressure  is  greatly  reduced,  such  rigidity  does  not 
exist,  the  fluid  state  of  the  material  being  clearly 
shown  by  the  lavas  from  volcanic  eruptions.  If 
the  entire  mass  of  the  terrestrial  spheroid  be  con- 
sidered to  be  solid,  the  phenomenon  of  latitude 
fluctuation  would  be  more  difficult  to  explain. 
Probably  there  is  a  circulation  of  these  internal 
fluid  parts.  Professor  Lagrange  of  Brussels  be- 
lieves he  has  found  a  relation  between  the  perio- 
dicity of  seismic  phenomena  and  that  of  polar 
movement. 

Chandler's  second  term  is  more  easily  explained 
since  the  period,  365  days,  is  an  annual  one.  Now 
meteorological  phenomena  show  a  similar  perio- 
dicity. Consider,  for  example,  the  displacement 
of  great  atmospheric  masses.  We  have  seen  that 
the  continents  are  chiefly  grouped  in  the  northern 
hemisphere;  during  the  winter  these  continents 
are  much  colder  than  the  oceans  which  surround 
them  and  consequently  the  land  is  covered  with  a 
layer  of  air  of  great  density.  The  total  mass  of 
air  thus  collected  in  winter  over  the  northern 
continents  is  in  excess  of  that  which  in  the  same 
season  covers  the  oceans.  Professor  Spitaler  has 
calculated  this  excess,  and  finds  it  to  be  fourteen 


120  THe  Earth 

thousand  million  tons,  that  is  to  day  it  equals  the 
weight  of  1500  cubic  kilometres  [930  cubic  miles] 
of  copper.  This  enormous  mass  moves  during 
the  summer  out  over  the  oceans,  and  it  is  quite 
possible  that  the  annual  periodical  displacement 
of  such  a  quantity  of  air  would  account  for  the 
periodicity  of  365  days  expressed  in  Chandler's 
formula.  We  must  also  add  that  the  periodical 
melting  of  the  polar  ice,  the  seasonal  variation  of 
rain  precipitation  and  other  phenomena  may  act 
in  the  same  way.  Dr.  Hahn  furthermore  believes 
that  an  annual  variation  of  the  Sun's  magnetic 
influence  exists.  On  the  whole,  therefore,  we  are 
in  possession  of  a  number  of  facts,  some  or  all  of 
which  may  explain  the  annual  periodicity  of  the 
polar  movement. 

In  a  general  way,  all  the  secular  phenomena 
which  occur  at  the  Earth's  surface  act  slowly  but 
cumulatively  upon  the  position  of  the  polar  axis; 
thus  erosion,  rising  and  sinking  of  the  land  surface, 
the  sudden  changes  produced  by  seismic  pheno- 
mena all  exert  influences  which  although  individ- 
ually feeble,  become  considerable  by  summation. 
Important  and  continued  geological  evolutions, 
such  as  those  which  have  marked  successive  eras 
in  the  Earth's  history,  must  have  had  marked 
effect  on  the  position  of  the  poles,  the  more  so  as, 


TKe  Movements  of  tKe  EartH      121 

in  primitive  ages,  the  crust  was  more  elastic  than 
it  is  at  the  present  time.  If  such  displacements 
were  produced  in  the  earlier  stages  of  the  Earth's 
history,  many  facts  known  to  geologists,  the  cause 
of  which  is  still  obscure,  would  be  explained  by 
them.  And  if  at  some  later  time  the  law  of  polar 
displacement  is  fully  elucidated,  by  means  of  the 
series  of  more  and  more  precise  observations  which 
scientists  will  accumulate  in  years  to  come,  it  is 
probable  that  much  light  will  be  thrown  upon  the 
past  history  of  our  globe. 

Thus,  the  poles,  the  conquest  of  which  has 
stimulated  so  much  noble  effort,  are  points  that  it 
is  impossible  to  fix.  Each  of  the  explorers  fortu- 
nate enough  to  have  hoisted  the  flag  of  his  country 
at  his  goal,  knew  that  on  the  next  day,  the  pole 
would  have  escaped  its  conqueror  and  would  oc- 
cupy some  other  nearby  part  of  the  surface. 

It  is  thus  not  possible  to  lay  hold  of  the  pole, 
to  speak  figuratively.  But  if  an  area  40  metres 
[130  ft.]  square  be  fenced  in  about  its  position  on 
any  given  day  it  may  be  affirmed  that  the  pole 
will  always  be  somewhere  inside  this  enclosure. 

We  have  studied  in  some  detail  the  irregularities 
to  which  the  Earth's  rotation  and  the  position  of 
its  axis  are  subject,  but  these  are  by  no  means  the 
only  ones.  The  movement  of  evolution  is  also 


122  The  Earth 

affected  by  perturbations,  with  regard  to  which  a 
few  words  will  now  be  said. 

In  the  first  place,  the  eccentricity  of  the  ellipse 
which  the  centre  of  the  Earth  describes,  in  accord- 
ance with  Kepler's  Laws,  varies;  it  diminishes 
uniformly  by  64  kilometres  [39.5  miles]  annually. 
Decreasing  eccentricity  implies  a  nearer  approach 
to  a  circular  form.  The  speed  of  revolution  round 
the  Sun,  which  Kepler's  Second  Law  states  is  not 
constant,  would  tend  to  become  more  and  more  so 
as  the  ellipse  approaches  the  circle.  If  the  above 
rate  of  decrease  were  to  continue,  the  ellipse 
would  actually  become  a  circle  in  about  40,000 
years,  with  the  result  that  the  Earth's  movement 
would  then  be  quite  uniform.1 

While  this  change  in  the  elliptical  form  of  the 
orbit  is  slowly  going  on,  the  ellipse  is  also  displaced 
in  its  plane  in  such  a  way  that  the  point  of  peri- 
helion moves  from  west  to  east  making  an  entire 
circuit  of  the  orbit  in  about  110,000  years,  thus 
introducing  a  second  irregularity  into  the  general 
movement  of  revolution.  Furthermore,  the  plane 
of  the  orbit,  to  which  that  of  the  terrestrial  equator 
is  inclined,  does  not  make  a  constant  angle  with 


1  The  gravitational  pull  of  the  other  planets  would  of  course 
continue  their  perturbing  effect  so  that  the  speed  of  the  Earth's 
revolution  could  not  become  really  constant. — Ed. 


THe  Movements  of  tHe  EartH      123 

the  latter.  This  angle  decreases  about  48  seconds 
per  century;  in  other  words,  the  planes  of  the 
equator  and  the  ecliptic  are  approaching  one 
another.  But  they  will  never  coincide,1  because 
the  orbital  plane  oscillates  backwards  and  forwards 
between  two  very  near  limits.  The  motion 
nevertheless  introduces  a  third  perturbation  into 
the  Earth's  movement  around  the  Sun.  Again, 
we  have  to  remember  that  the  Earth  does  not 
revolve  alone  around  the  Sun.  It  is  accompanied 
by  a  small  companion  or  satellite,  the  Moon, 
whose  mass  is  ^  that  of  the  Earth  and  whose  dis- 
tance from  us  is  equal  to  sixty  times  the  Earth's 
radius.  Strictly  speaking  it  is  not  the  Earth, 
but  the  complex  Earth-Moon  system,  which 
revolves  around  the  Sun.  Now  the  duality  of  the 
system  necessitates  that,  if  the  Moon  describes 
a  certain  ellipse  around  the  Earth,  the  Earth,  whose 
mass  is  eighty  times  greater,  describes  an  ellipse 
eighty  times  smaller.  The  result  is  exactly  as  if, 
having  attached  two  balls  to  the  extremities  of  a 
string,  one  being  eighty  times  heavier  than  the 
other,  the  system  was  thrown  forth  into  the  air 
so  that  the  string  remained  stretched;  the  two 


1  Some  modern  astronomers  believe  that  they  will;  that 
eventually  all  the  planets  will  revolve  with  their  axes  perpen- 
dicular to  their  orbital  planes. — Ed. 


124  The  Earth 

balls  would  thus  be  obliged  to  take  a  common 
movement  of  revolution  and,  at  the  same  time, 
they  each  would  revolve  about  a  point  situated 
on  the  line  joining  the  centres  of  the  balls,  8-0 
of  the  distance  from  the  centre  of  the  large  ball 
to  that  of  the  smaller. 

In  the  particular  case  we  are  dealing  with,  that 
of  the  Moon  and  the  Earth,  the  force  of  attraction 
reciprocally  exerted  between  the  two  masses,  in 
accordance  with  Newton's  Law,  takes  the  place 
of  the  material  connection.  Consequently,  it  is 
the  centre  of  gravity  of  the  Earth-Moon  system 
which  describes  the  elliptical  orbit  round  the  Sun. 
As  the  masses  and  distance  apart  of  the  two  bodies 
are  known,  it  may  be  shown  that  the  centre  of 
gravity  lies  on  the  line  joining  the  centres  of  the 
Earth  and  Moon,  about  1000  kilometres  [620 
miles]  below  the  surface  of  the  former.  It  is, 
thus,  a  point  in  the  interior  of  the  terrestrial  globe, 
and,  as  it  is  markedly  eccentric,  it  produces 
another  anomaly  in  the  Earth's  movement  around 
the  Sun. 

Finally,  there  is  yet  another  movement  super- 
imposed upon  all  the  others;  it  is  a  general  move- 
ment affecting  the  whole  Solar  System  which  is 
traversing  intersidereal  space,  moving  in  the 
general  direction  of  the  star  Vega  with  a  consider- 


THe  Movements  of  tHe  EartK      125 

able  velocity,  viz.,  more  than  20  kilometres  [12 
miles]  per  second.  By  reason  of  this  general 
movement  of  translation,  the  Earth's  elliptical 
orbit  is  actually  an  immense  elliptical  spiral,  like 
a  screw-thread  whose  diameter  is  the  major  axis 
of  the  terrestrial  orbit,  that  is  to  say,  more  than 
297  million  kilometres  [185,450,000  miles].  The 
step  of  the  thread,  the  distance  the  Sun  and 
whole  Solar  System  move  in  the  course  of  a  year, 
is  more  than  627  million  kilometres  [388,750,000 
miles].  The  point  of  the  sky  towards  which  this 
journey  appears  to  be  directed  is  called  the  apex. 

The  fact  of  this  movement,  and  its  measurement, 
have  been  achieved  by  a  very  beautiful  applica- 
tion of  the  theory  of  the  propagation  of  vibratory 
movement,  an  application  which  Do'ppler  and 
Fizeau  worked  out  in  principle  and  which  modern 
astrophysicists  such  as  Deslandres  and  Hale  have 
put  into  practice  with  notable  results.  When  a 
luminous  source  is  in  motion  in  the  direction  along 
which  we  see  it,  that  is  to  say  when  it  comes  either 
directly  towards  us,  or  moves  directly  away  from 
us,  in  a  straight  line,  the  velocity  of  the  light 
reaching  us  from  the  source  is  affected  by  the 
actual  velocity  with  which  the  source  is  moving. 
Consequently,  if  the  light  coming  from  this  mov- 
ing source  be  received  in  a  spectroscope,  the  rays 


126  The  EartH 

of  the  spectrum  will  be  displaced  either  towards 
the  violet  end  or  the  red  end,  according  to  whether 
the  source  is  approaching  us  or  receding  from  us. 
Precisely  the  same  thing  occurs  if  the  eye  of  the 
observer  is  in  motion  relative  to  a  fixed  source, 
or,  again,  if  the  two  are  relatively  displaced  in  any 
way.  In  all  cases  a  displacement  towards  the 
violet  indicates  that  the  distance  between  the 
eye  and  the  source  is  diminishing;  one  towards 
the  red  indicates  the  reverse,  viz.,  that  the  distance 
is  increasing. 

The  theoretical  explanation  is  the  same  as  that 
which  obtains  with  regard  to  the  phenomenon 
observed  in  the  case  of  a  whistling  locomotive; 
the  pitch  of  the  note  varies  according  as  the 
locomotive  is  coming  towards  or  receding  from  us. 

The  Earth,  as  a  whole,  has  therefore  twelve 
different  movements  which  science  has  been  able 
to  analyse,  and  of  which  the  effects  have  been 
studied  and  the  causes  discovered.  We  shall  see 
later  that,  not  in  its  entirety  but  as  regards  its 
crust  alone,  the  Earth  is  subject  to  other  move- 
ments of  astronomical  origin ;  these  are  the  oceanic 
tides,  and  the  terrestrial  tides,  which  give  to  the 
ground  a  perpetual  movement  of  exceedingly  great 
complexity,  its  apparent  stability  being  only  an 
illusion. 


CHAPTER  V 

GRAVITY 

one  of  the  mechanical  phenom- 
ena,  one  of  the  manifestations  of  movement, 
which  most  strikes  the  least  experienced  observer 
is  the  fall  of  bodies.  When  a  material  body, 
which  has  been  raised  to  a  certain  height,  is 
deprived  of  its  support,  it  falls  to  the  ground, 
following  the  line  which  joins  the  original  position 
of  the  body  to  the  centre  of  the  Earth,  at  least  as 
nearly  as  our  senses  and  instruments  can  show. 

This  line  is  called  the  vertical  of  the  place. 
All  bodies  obey  this  law  of  falling,  which  is  the 
law  of  gravity.  Consequently,  liquids  by  reason 
of  their  fluidity  dispose  themselves  so  that  their 
free  surfaces  are  at  each  point  normal  to  the  verti- 
cal at  the  point  in  question.  Such  surfaces  are 
therefore  curvilinear,  and  their  formation  is  due 
to  the  combined  action  of  the  laws  of  gravity  and 
centrifugal  force  which  give  to  the  Earth  its  ellip- 
soidal form.  The  surface  of  the  oceans  imaginarily 

127 


128  The  Earth 

prolonged  under  the  continents  is,  as  we  have 
seen,  called  the  geoid.  If  it  be  of  very  small 
extent,  the  curvilinear  surface  of  a  liquid  mass 
coincides  with  its  tangent  plane  and  in  this  case 
only  does  it  form  a  horizontal  plane  perpendicular 
to  the  vertical  of  the  place. 

Gravity  thus  furnishes  us  with  the  data  as  to 
our  fundamental  directions  of  horizontality  and 
verticality.  If  we  suspend  a  heavy  body  on  a 
string  attached  to  a  fixed  point,  the  string,  being 
flexible,  takes  the  direction  along  which  the  body 
is  drawn  towards  the  Earth.  It  forms  a  plumb 
line  which  indicates  the  vertical  of  the  place. 

The  weight  of  a  body,  which  makes  it  fall  to- 
wards the  Earth,  is  a  particular  case  of  the  univer- 
sal attraction  between  portions  of  matter;  it  is 
not  a  distinct  kind  of  force.  The  attractive  force 
between  two  bodies  is  proportional  to  the  product 
of  masses  and  in  inverse  ratio  to  the  square  of 
their  distances.  In  the  case  we  are  considering, 
the  Earth  is  one  of  the  bodies,  viz.,  that  with  the 
preponderating  mass.  Since  it  is  nearly  spherical, 
its  effect,  on  anything  outside  it,  is  the  same  as  if 
its  entire  mass  was  accumulated  at  its  geometrical 
centre.  The  other  body,  that  which  is  attracted, 
is  the  one  which  falls  to  the  ground.  We  have 
thus,  under  our  very  eyes,  an  illustration  of  the 


Gravity  129 

law  which  Newton  enunciated,  and  which  governs 
the  movements  of  the  bodies  in  infinite  space. 

Father  Ximenes,  as  early  as  1757,  had  pointed 
out  that  the  balance  could  demonstrate  the  iden- 
tity of  weight  and  gravitation,  but  lack  of  precision 
of  the  instruments  of  his  period  prevented  him 
from  realising  his  idea.  The  experiment  devised 
by  the  Spanish  scientist  was  carried  out  by  Jolly 
towards  the  end  of  the  nineteenth  century.  The 
essentials  are  as  follow:  A  balance  is  placed  on 
an  elevated  support,  such  as,  for  example,  the 
flooring  of  a  higher  storey  of  a  house,  and  a  long 
fine  metallic  wire  is  suspended  from  below  one  of 
its  scale-pans.  On  the  latter,  say  the  right  pan, 
a  weight  of  one  kilogram  [2  Ib.  3  oz.  4  dr.]  is 
placed,  while  on  the  other,  the  left  pan,  a  similar 
weight  is  put  to  give  equilibrium.  If  the  weight 
be  now  hung  on  the  end  of  the  wire,  after  removal 
from  the  pan  where  it  formerly  was,  the  other 
weight  being  untouched,  the  equilibrium  is  de- 
stroyed because  the  first  weight  is  now  nearer  to 
the  centre  of  the  Earth,  and  so  is  attracted  with 
greater  force. 

It  is  easy  to  calculate  the  difference;  for  a 
height  of  300  metres  [990  ft.],  such  as  that  of  the 
Eiffel  Tower,  the  variation  is  10.000  of  the  weight. 
In  other  words  if  the  above  experiment  were 

9 


130  TKe  EartK 

made  from  the  height  of  the  Tower  with  the 
weight  of  one  kilogram  suspended  at  the  end 
of  the  wire  near  the  ground,  it  would  be  necessary 
to  add  one  decigram  [1.54  gr.]  to  the  other  pan  to 
restore  equilibrium.  For  a  height  of  30  metres 
[99  ft.],  one  centigram  [.154  gr.]  would  be  required, 
and  for  one  of  3  metres  [9.9  ft.]  the  difference  would 
be  one  milligram  [.015  gr.]  Now,  the  sensitive 
balances  to  be  found  in  modern  physical  laborato- 
ries will  easily  weigh  a  kilogram  to  within  ^  of 
a  milligram.  The  experiment  may  thus  be  easily 
carried  out  between  the  ceiling  and  floor  of  a 
room;  it  is  extremely  instructive,  and  should  be 
done  in  schools  and  colleges  at  the  beginning  of 
every  course  of  physics. 

Once  it  is  realised  that  weight  is  identical  with 
universal  gravitation,  it  will  be  seen  that  it  is  not 
strictly  correct  to  say,  as  is  done  in  courses  of 
elementary  physics,  that  "gravity  is  a  force 
constant  in  magnitude  and  direction."  It  is  not 
constant  in  magnitude,  for  it  varies  with  the  least 
vertical  displacement,  and  we  shall  see  that  it 
also  varies  with  the  least  horizontal  displacement; 
neither  is  it  constant  in  direction,  since  it  is  directed 
along  the  vertical,  and  two  neighbouring  verticals 
meet  about  the  centre  of  the  Earth.  And,  if 
within  the  limits  even  of  a  room  it  is  possible  to 


Gravity  131 

demonstrate  a  variation  in  its  intensity,  it  would 
be  equally  possible  to  prove  astronomically,  by 
means  of  the  meridian  circle  and  variation  in  the 
direction  of  the  vertical  with  respect  to  the  celestial 
sphere,  in  the  same  space. 

The  laws  of  the  fall  of  bodies  given  in  similar 
courses  of  physics  are  similarly  not  exact.  The 
law  of  velocities  and  the  law  of  distances  are  veri- 
fied with  an  apparatus  called  Atwood's  machine, 
generally  2  metres  [6.5  ft.]  in  height.  Now, 
Jolly's  experiment  succeeds  with  a  difference  of 
height  of  two  metres;  it  indicates  a  difference  of 
two- thirds  of  a  milligram  [.oio  gr.]  for  the  weight 
of  one  kilogram  [2  Ib.  3  oz.  4  dr.]  transferred 
from  the  top  of  the  apparatus  to  the  bottom. 
Therefore,  if  the  experiment  with  Atwood's 
machine  appears  to  succeed,  it  is  thanks  to  the 
systematic  lack  of  precision  of  the  apparatus,  to 
the  unconscious  assistance  of  the  experimenter, 
and  the  inexperience  of  his  pupils. 

On  the  other  hand,  we  may  demonstrate  for 
purposes  of  instruction  the  existence  of  gravitation 
by  that  of  weight;  if  the  story  be  true,  Newton  in 
watching  the  fall  of  an  apple  had  the  first  inspira- 
tion of  his  discovery  of  the  law  which  governs  the 
movements  of  the  celestial  bodies  in  space. 

The  variation  of  the  force  and  the  direction  of 


132  TKe  Earth 

gravity  demands  methods  and  instruments  of 
exceptional  precision  for  its  measurement. 

The  determination  of  the  direction  of  gravity 
resolves  itself  into  the  determination  of  the  true 
vertical  at  each  point  on  the  Earth's  surface. 
This  vertical  is  normal  to  the  ideal  surface  which 
is  called  the  geoid,  formed,  as  before  stated,  by 
the  prolongation  of  the  oceanic  surface  underneath 
the  emergent  land  masses.  The  determination 
of  the  direction  of  gravity  therefore  necessitates 
the  study  of  the  exact  form  of  the  geoid,  that  is  to 
say,  the  form  of  the  Earth  itself.  A  special  science, 
geodesy,  deals  with  this  matter  and  we  shall  shortly 
have  to  return  to  its  methods  more  fully. 

The  measurement  of  the  intensity  of  gravity 
at  a  given  place  is  a  problem  in  mechanics.  Since 
gravity  is  a  force,  we  may  study  that  force  by  the 
effects  which  it  produces;  there  are  two  distinct 
ways  of  doing  this,  the  dynamic  method  and  the 
static  method.  The  first  consists  in  studying 
the  movement  impressed  on  a  given  system  by  the 
action  of  the  force  in  question;  in  the  second  we 
maintain  a  state  of  equilibrium  of  the  body,  which 
is  submitted  to  the  action  of  the  force  which  we 
require  to  measure  by  counterbalancing  it  with 
another  force,  the  value  of  which  is  known. 

The  dynamic  method  has  been,  up  to  the  pre- 


Gravity  133 

sent,  the  one  almost  wholly  employed,  and  the 
only  form  which  this  method  takes  is  that  of 
oscillations.  When  a  body  capable  of  oscillation, 
such  as  a  magnetised  needle,  for  example,  be  dis- 
placed from  its  position  of  equilibrium,  it  tends 
to  revert  to  it,  and  executes  a  series  of  oscillations, 
the  amplitude  of  which  decreases  logarithmically. 
If  we  place  the  pole  of  an  electromagnet  near  this 
needle,  the  latter  will  execute  a  certain  succession 
of  oscillations,  which  can  be  determined  by  obser- 
vation. Now,  if  the  intensity  of  the  current  which 
circulates  around  the  iron  nucleus  of  the  electro- 
magnet be  increased  and  the  intensity  of  the 
magnetic  force  due  to  it  consequently  also  aug- 
mented, the  needle  will,  under  the  influence  of 
the  greater  force,  oscillate  more  and  more  quickly 
in  proportion  as  the  force  increases. 

This  is  exactly  analogous  to  the  method  used 
in  studying  gravity.  A  heavy  body  is  taken, 
suspended  at  the  extremity  of  a  fine,  but  inextens- 
ible,  thread.  This  constitutes  a  plumb  line.  The 
body  is  displaced  from  its  equilibrium  position 
and  left  to  itself;  under  the  action  of  gravity, 
which  tends  to  bring  it  to  a  position  as  near  as 
possible  to  the  Earth's  centre,  it  executes  a  series 
of  oscillations  of  gradually  decreasing  amplitude, 
thus  constituting  a  pendulum.  Should  the  in  ten- 


134  The  EartH 

sity  of  gravity  increase,  the  oscillations  will  be 
more  rapid  and,  on  the  other  hand,  in  a  place 
where  the  intensity  is  feebler,  they  will  be  slower. 

This  apparatus  would  be  what  mathematicians 
call  a  simple  pendulum.  If  the  thread  were  free 
from  friction  at  its  point  of  suspension,  if  it  were 
without  mass,  while  still  remaining  rigid  and  in- 
extensible,  and  if  the  suspended  body  had  no 
dimensions  but  were  merely  a  heavy  material 
point,  the  law  governing  its  movement  would  be 
also  simple;  the  oscillations,  while  subjected  to  a 
logarithmic  decrement,  would  continue  indefinitely. 
When  they  became  of  extremely  small  amplitude, 
they  would  be  isochronous,  and  their  period  would 
be  proportional  to  the  square  root  of  the  length 
of  the  thread,  and  inversely  proportional  to  the 
square  root  of  the  intensity  of  gravity  at  the  place 
where  the  pendulum  is. 

Unfortunately,  this  ideal  pendulum  is  absolutely 
unrealisable  in  practice;  however  fine  the  thread 
may  be,  it  has  some  mass;  the  body  which  is  sus- 
pended on  it,  and  which  is  usually  in  the  form  of  a 
ball,  has  dimensions  and  so  cannot  be  considered 
as  a  mathematical  point  however  great  its  density 
may  be.  Furthermore,  whether  the  thread  be 
suspended  from  a  knife-edge  or  held  in  a  vice, 
friction  is  bound  to  come  into  play ;  also  the  whole 


Gravity  135 

oscillates  in  a  resisting  medium.  For  these  reasons 
the  experiment  so  beautifully  simple  in  principle 
is  extremely  difficult  to  carry  out  actually. 

Nevertheless,  Bouguer  and  La  Condamine 
attempted  to  measure  the  intensity  of  gravity 
with  a  pendulum  approaching  as  nearly  as  possible 
to  the  simple  pendulum,  but  it  is  Borda  to  whom 
the  credit  is  due  of  making  the  first  really  precise 
experiment,  from  which  the  law  of  the  oscillations 
and  the  value  of  gravity  at  a  given  place  could 
be  deduced.  The  famous  sailor  tried  to  realise 
as  far  as  possible  the  conditions  of  the  simple 
pendulum.  For  the  heavy  body  he  used  a  plati- 
num ball,  the  high  density  of  which  enabled  it  to 
be  of  relatively  small  size;  the  suspending  wire 
was  also  of  platinum  and  hung  from  the  knife- 
edge  of  a  balance.  The  wire  carried  at  its  lower 
end  a  hollow  greased  metallic  cap  to  which  the 
ball  of  platinum  was  fixed  by  simple  adherence. 
The  whole  was  therefore  composed  of  two  parts: 
the  ball,  and  the  knife-edge  wire-cap  system. 
Borda  measured  the  duration  of  oscillation  of  the 
complete  pendulum;  then,  removing  the  ball,  he 
displaced,  by  means  of  a  screw  with  a  heavy  head, 
the  centre  of  gravity  of  the  knife  and  increased 
this  displacement  until  the  knife-edge  wire-cap 
system,  oscillating  as  a  pendulum,  had  the  same 


136  The  EartH 

period  of  oscillation  as  the  original  pendulum  fur- 
nished with  the  ball.  In  these  circumstances,  the 
suspension  system  did  not  enter  into  the  calcula- 
tion at  all,  and  the  result  was  as  if  Borda  was  using 
a  pendulum  constituted  of  a  wire  without  mass 
supporting  a  heavy  sphere.  Now  the  formulae  of 
mechanics  enable  one  to  calculate  the  moment  of 
inertia  of  a  sphere  with  respect  to  an  exterior  axis 
around  which  it  oscillates ;  the  problem  was  thus 
solved,  save  for  the  corrections  due  to  the  per- 
turbing action  of  air-currents.  Borda  had  pre- 
viously devised  a  formula  which  allowed  for  the 
amplitude  of  the  oscillations  when  these  were  not 
extremely  small. 

This  method  has  been  abandoned,  though  it  is 
difficult  to  assign  a  reason  for  this ;  it  only  requires 
the  measurement  of  the  duration  of  oscillation, 
a  measurement  which  is  equally  necessary  in  all 
cases,  whatever  form  of  pendulum  method  be 
employed,  and  also  the  measurement  of  the  dis- 
tance between  the  knife-edge  and  the  centre  of  the 
ball.  It  allows  of  moving  the  heavy  ball  with 
reference  to  the  cap  in  which  it  fits,  by  twisting 
it  round,  and  so  eliminating  any  error  due  to  the 
non-homogeneity  of  the  ball,  the  mean  of  experi- 
ments with  different  positions  being  taken.  The 
whole  experiment  may  be  carried  out  in  vacuo, 


Gravity  137 

and,  consequently,  it  follows  that  it  is  susceptible 
of  the  maximum  degree  of  precision  that  we  can 
attain  at  the  present  time. 

In  spite  of  this  fact,  modern  geodesists  have 
given  up  the  simple  pendulum  method  and  only 
use  that  of  the  compound  pendulum. 

The  compound  pendulum  consists  merely  of 
any  body  whatsoever,  which  is  caused  to  oscil- 
late about  any  axis  not  passing  through  its  centre 
of  gravity.  Under  the  action  of  gravity  the  body 
originally  takes  up  a  position  of  equilibrium,  and, 
when  it  is  displaced  from  this  position,  it  executes 
a  series  of  oscillations  according  to  the  pendulum 
law.  But,  even  if  the  oscillating  body  had  some 
definite  geometrical  form,  the  experiment  would 
hardly  be  suitable  for  the  precise  determination 
of  the  intensity  of  gravity  if  De  Prony,  the  inventor 
of  the  dynamometer  which  bears  his  name,  had 
not  discovered  a  curious  and  unexpected  property 
characterising  the  compound  pendulum. 

Let  us  take  a  body  of  any  form  whatever.  Near 
one  of  its  extremities  we  fix  a  transverse  axis  and 
make  the  body  oscillate  about  this  axis,  which  is 
called  the  axis  of  suspension.  These  oscillations 
would  have  a  definite  period  which  is  measured 
and  noted  with  care.  We  next  make  use  of  a 
second  axis  parallel  to  the  first  one  and  placed  in 


138  The  Earth 

such  a  way  that  if  the  body  be  caused  to  oscillate 
round  this  new  axis,  to  which  the  name  axis  of 
oscillation  is  given,  the  new  period  of  oscillation 
is  exactly  the  same  as  the  former  one.  The  science 
of  mechanics  shows  that  it  is  always  possible  to 
find  the  point  where  this  second  axis  must  be 
placed,  in  any  body;  there  are  actually  several 
such  places.  The  two  axes,  those  of  suspension 
and  oscillation,  are  reciprocal  to  one  another. 

This  being  done,  it  may  be  proved  that  when 
the  axes  have  been  adjusted  as  above  described, 
the  distance  between  them  is  equal  to  the  length 
of  the  simple  pendulum  that  would  have  the  same 
period  of  oscillation.  This  is  the  salient  feature 
of  Prony's  discovery. 

The  importance  of  this  will  now  be  seen.  The 
simple  pendulum,  unrealisable  as  such,  is  indirectly 
realised  by  means  of  the  compound  pendulum. 
All  that  is  necessary  is  to  find  by  repeated  experi- 
mental trials  the  position  of  the  two  axes,  to 
measure  the  distance  between  them  with  all  possible 
precision,  and  to  determine  the  period  and  ampli- 
tude of  the  oscillations  of  the  body.  The  value 
of  the  intensity  of  gravity  at  the  place  where  the 
experiment  is  made  may  be  deduced  from  the 
formula  for  the  simple  pendulum. 

Although  the  principle  of  this  method  is  not 


Gravity  139 

complicated,  its  practice  is  a  very  delicate  matter 
on  account  of  the  precision  which  is  necessary, 
and  which  exceeds  30olooo  part. 

In  the  first  place,  it  is  necessary  to  correct  the 
pendulum  for  the  variations  in  length  due  to 
changes  in  the  temperature  of  its  surroundings. 
Then  there  is  the  influence  of  the  nature  of  the 
medium  in  which  the  body  oscillates,  that  is  to 
say,  the  disturbing  effects  of  the  air. 

One  effect  which  the  air  has  on  the  pendulum 
is  to  exert  a  thrust  upon  it,  lessening  its  apparent 
weight,  according  to  the  principle  of  Archimedes, 
with  the  result  that  the  pendulum  oscillates  a 
little  more  easily  under  the  action  of  a  given  force 
than  if  the  same  experiment  were  conducted  in 
vacuo.  Again,  it  offers  resistance  to  the  move- 
ment of  the  apparatus,  a  resistance  which  affects 
all  moving  bodies.  This  is  easily  recognised  by 
artillerymen  and  by  cyclists,  and  it  is  on  account 
of  such  resistance  that  birds  and  aeroplanes  can 
move  through  the  atmosphere.  The  degree  of 
resistance  increases  very  rapidly  in  proportion  as 
the  velocity  of  the  body  in  question  is  augmen- 
ted. In  the  particular  case  of  the  pendulum, 
which  oscillates  slowly,  it  is  very  feeble,  but  never- 
theless not  negligible  when  the  degree  of  precision 
we  require  is  taken  into  account.  Furthermore, 


140  The  EartH 

there  is  a  third  effect ;  the  air  is  to  a  certain  extent 
carried  along  with  the  moving  pendulum,  and  from 
this  it  follows  that  the  loss  of  weight  due  to  the 
atmospheric  thrust  above  mentioned  is  doubled. 
Finally,  as  the  air  is  far  from  being  a  perfect 
fluid,  it  possesses  a  certain  viscosity  and  this 
viscosity  helps  to  retard  the  oscillatory  movement 
of  the  pendulum. 

These  complex,  and  by  no  means  negligible, 
effects  of  the  air  may  be  eliminated  by  making 
the  pendulum  oscillate  in  a  vacuum,  a  procedure 
which  has  only  recently  been  carried  out  in  practice. 

The  determination  of  the  length  of  the  pend- 
ulum, that  is  to  say  the  distance  separating  the 
two  parallel  axes,  is  a  delicate  operation.  These 
axes  are  represented  by  two  knife-edges,  the  edges 
being  turned  towards  one  another.  It  is  possible 
to  obtain  the  value  of  this  distance  to  within  a 
micron.1 

The  measurement  of  the  duration  of  the  oscil- 
lations is  an  even  more  delicate  matter;  it  neces- 
sitates the  determination  of  a  period  of  about  one 
second  with  a  precision  of  the  same  order  of 
magnitude  as  that  which  we  wish  to  attain  in  the 
resulting  value  of  gravity. 

An  idea  which  readily  occurs  to  one  is  to  measure 

1 A  micron  (/*)  =TWO  of  a  millimetre  =  .00003937  in. 


Gravity  141 

by  means  of  an  astronomical  clock,  regulated  to 
keep  sidereal  time,  the  duration  of  say  1000 
oscillations  and  to  divide  this  quantity  by  1000. 
This  is  the  method  of  passages;  it  is  long  and 
tedious  and  tends  to  tire  the  observer  and  cause 
him  to  make  large  errors.  It  would  be  possible 
to  re-introduce  this  method  at  the  present  time, 
registering  the  oscillations  by  photography  on 
cinematograph  films  and  perhaps  a  very  good 
result  could  be  thus  obtained. 

The  other  artifices  are  preferred:  the  method 
of  coincidences  devised  by  Mairan,  and  the  method 
of  phases  applied  by  the  Austrian  general,  von 
Sterneck.  The  latter  is  the  one  now  most 
employed. 

The  pendulum  may  be  used  to  give  information 
of  two  different  kinds,  either,  to  furnish  the  ab- 
solute value  of  the  intensity  of  gravitation  at  a 
given  place,  or  to  give  the  relative  value  of  the 
intensities  at  different  stations  on  the  Earth's 
surface. 

The  absolute  measurement  is  difficult  as  we 
have  already  seen,  since  it  implies  the  determina- 
tion of  a  length  and  an  interval  of  time  with  the 
greatest  precision.  The  relative  measurement  is 
easier  and  thanks  to  the  method  instituted  by 
General  von  Sterneck  it  is  now  in  current  use. 


142  The  Earth 

It  consists  in  taking  a  pendulum,  which  of 
course  remains  of  invariable  dimensions,  and  caus- 
ing it  to  oscillate  successively  at  two  distinct 
stations,  in  identical  conditions,  measuring  in 
each  case  the  duration  of  the  oscillation.  The 
formula  for  the  simple  pendulum  shows  that  the 
intensities  of  gravity  at  the  two  stations  are  in 
the  inverse  ratio  of  the  squares  of  the  oscillation 
periods. 

This  being  so,  it  suffices,  in  order  to  determine 
the  absolute  value  of  gravity  at  various  places  on 
the  Earth's  surface,  to  measure  it  absolutely  at 
any  one  place,  Paris  for  example;  then  the  same 
pendulum  is  taken  to  the  required  places  and  the 
values  relative  to  that  at  Paris  obtained.  In  this 
way  a  map  showing  the  values  at  different  places 
may  be  made. 

Although  the  method  used  for  this  practical 
application  of  the  pendulum  to  the  determination 
of  gravity  is  a  very  good  one,  nevertheless  it 
requires  a  series  of  complex  operations  and  long 
and  delicate  manipulations.  Also,  the  measure- 
ments are  difficult,  and  the  apparatus  fragile  and 
bulky.  An  astronomical  clock  is,  in  fact,  neces- 
sary, as  is,  also,  an  instrument  to  observe  the 
stars  and  so  regulate  the  clock  to  keep  sidereal 
time.  Then  there  is  the  pendulum,  or  rather 


Gravity  143 

pendulums  (von  Sterneck  used  four),  a  firm  sup- 
port to  hold  them  and  an  air-pump  and  receiver 
to  create  a  vacuum  in  which  to  place  them. 
Finally,  an  apparatus  is  necessary  to  measure  the 
coincidences,  and  the  whole  constitutes,  as  will 
be  seen,  a  complicated  arrangement,  exacting 
as  regards  the  personnel  and  necessitating  an 
expenditure  of  much  time. 

Physicists  have  therefore  endeavoured  to  find 
if  it  would  not  be  possible  to  measure  the  intensity 
of  gravity  directly  by  a  static  method,  attaining 
equilibrium  between  gravity  which  tends  to  draw 
a  heavy  body  to  the  ground  and  an  opposing  force, 
which  is  known  or  measurable,  equal  and  of 
contrary  sign.  The  principle  of  such  an  arrange- 
ment is  attractive  because  if  it  could  be  realised 
with  the  necessary  precision,  the  intensity  of 
gravity  at  a  place  could  be  deduced  by  a  simple 
reading  of  the  graduation. 

The  most  simple  kind  of  such  an  apparatus  is 
the  spring  balance.  The  elasticity  of  stretching 
of  a  spiral  spring  depends  only  on  the  nature  of 
the  metal  and  not  on  the  value  of  gravity.  If, 
therefore,  a  very  sensitive  balance  be  made  use 
of  and  taken  to  different  places,  the  same  body 
being  always  suspended  from  it,  the  body  will 
appear  to  weigh  more  or  less  according  as  the 


144  The  Earth 

intensity  of  gravity  at  the  place  in  question  is 
stronger  or  weaker.  Consequently  the  spring  is 
stretched  to  various  extents  in  the  different  parts 
of  the  globe  to  which  it  is  taken  and  the  variations 
in  length  give  us  the  relative  values  of  gravity 
in  these  places. 

But  it  is  a  far  cry  from  theory  to  practice  and 
the  good  spring  gravity  measuring  machine  has 
yet  to  be  constructed.  However,  Threlfall  has 
made  an  instrument  which  has  a  high  degree  of 
sensitiveness;  he  does  not  employ  the  property  of 
stretching  but  that  of  the  torsion  of  a  very  fine 
thread  of  quartz,  and  geo-physicists  anticipate 
that  they  will  be  able  to  do  good  work  with  it. 

An  elasticity  to  which  scientists  have  for  a 
long  time  given  their  attention  is  that  of  a  gas 
forced  to  occupy  a  constant  volume  by  the  pres- 
sure of  a  column  of  mercury.  The  mercury,  the 
weight  of  which  is  responsible  for  the  pressure  to 
which  the  gas  is  submitted,  weighs  more  or  less 
according  to  the  value  of  the  intensity  of  gravity 
at  the  place  of  the  experiment.  The  result  is, 
therefore,  that  we  balance  a  pressure,  which  always 
remains  the  same,  with  mercury  columns  of  differ- 
ent densities  of  which  the  heights  will  thus  be 
in  inverse  ratio  to  the  densities,  that  is  to  say 
to  the  corresponding  intensities  of  gravity.  The 


Gravity  145 

method  is  simple  and  ingenious  but  unfortunately 
the  great  sensitiveness  of  the  pressure  of  the 
gas  to  variations  of  temperature  (2-7-3  part  per 
degree  Centigrade)  renders  the  method  very 
difficult  in  actual  practice. 

Count  WullersdorfT-Urbair,  of  Vienna,  has 
attempted  to  avoid  this  difficulty  by  only  making 
use  of  a  gaseous  mass  as  an  intermediary  between 
two  manometric  arrangements,  one  of  which 
depends  on  gravity  while  the  other,  serving  as  a 
standard,  does  not  vary  with  anything.  The 
mass  of  gas  chosen  for  this  purpose  is  that  of  the 
atmosphere  itself.  Let  us  imagine  that  the  pres- 
sure of  the  atmosphere  is  measured  simultaneously 
by  means  of  two  barometers,  one  a  mercury  baro- 
meter and  the  other  a  spring  barometer,  such  as 
an  aneroid.  At  the  first  station,  the  two  instru- 
ments are  made  to  agree,  but  this  agreement  will 
not  hold  good  at  a  second  station,  for  which 
gravity  has  a  different  value.  In  fact,  the  mercury 
weighs  more  in  the  place  where  the  force  of  gravity 
is  stronger.  It  is  then  denser,  and  in  this  denser 
condition  a  column  of  mercury  of  less  height  will 
balance  a  pressure  which  would  have  necessitated 
a  longer  column  of  mercury  of  normal  density. 
The  aneroid  barometer  should  always  indicate, 
on  the  contrary,  the  true  pressure.  The  greater 

M 


146  The  Earth 

or  less  extent  of  disagreement  between  the  two 
instruments  enables  the  variations  of  gravity  to 
be  deduced,  the  atmospheric  pressure  acting  only 
as  an  intermediary  agent. 

The  method  appears  good  but  it  is  vitiated  by 
a  weak  point,  viz.,  the  aneroid  barometer.  De- 
pending upon  the  elasticity  of  a  spring  it  is  subject 
to  vicissitudes;  the  elasticity  of  steel  changes 
greatly  with  temperature  and  also  slowly  varies 
with  time.  The  method,  therefore,  would  not  have 
been  susceptible  of  the  necessary  precision  if  the 
Swiss  physicist  Guillaume  had  not  conceived  the 
idea  of  replacing  the  aneroid  barometer  by  an 
instrument  called  the  hypsometer,  which  gives 
the  value  of  the  atmospheric  pressure  by  deter- 
mining the  value  of  the  boiling  point  of  water, 
which  depends  on  that  pressure.  The  tables  of 
boiling  points  and  the  respective  atmospheric 
pressures  they  correspond  to  are  drawn  up  for 
the  normal  values  of  the  pressures,  measured  by 
a  column  of  mercury  at  zero  Centigrade,  at  sea 
level,  in  a  definite  latitude  on  the  Earth's  surface, 
viz.,  that  of  45°.  So  that  if  the  indications  of  the 
barometer  and  hypsometer  do  not  agree,  the  differ- 
ence gives  the  variation  of  gravity.  The  hypso- 
meter has  been  brought  to  a  high  state  of  perfection, 
due  to  the  progress  of  thermometry.  Professor 


Gravity  147 

Mohn  of  Christiania  has  made  use  of  the  method 
with  great  success  on  land,  and  the  German 
geodesist,  Dr.  Hecker,  has  attempted  to  utilise 
it  in  the  course  of  two  voyages,  one  in  the  Pacific 
and  one  in  the  Atlantic,  in  order  to  obtain  the 
value  of  gravity  on  the  open  sea,  where  the  em- 
ployment of  a  pendulum  is  quite  impossible.  The 
maximum  degree  of  precision  which  may  be 
obtained  by  this  method  appears  to  be  one 
part  in  50,000,  and  this  is  amply  sufficient  to 
detect  certain  anomalies  in  the  normal  value  of 
gravity. 

In  a  general  way  all  physical  phenomena,  in 
the  analytical  formula  for  which  occurs  the  value 
of  the  intensity  of  gravitation,  the  symbol  for 
which  is  g  may  be  utilised  to  determine  this  quan- 
tity. Thus,  the  fall  of  bodies,  the  velocity  of 
sound,  and  the  pitch  of  the  musical  note  emitted 
by  a  vibrating  wire  stretched  by  means  of  a 
weight  on  a  sounding  box  will  give  data  from  which 
the  value  of  g  may  be  deduced.  The  value  of  g 
also  enters  into  many  phenomena  in  connection 
with  wave  systems.  The  velocity  of  propagation 
of  a  seismic  wave  of  translation  over  the  surface 
of  a  large  ocean  is  a  function  of  the  depth  of  the 
ocean  and  the  mean  value  of  the  intensity  of 
gravitation  at  its  surface.  Furthermore,  all  ex- 


148  The  Earth 

periments  which  have  to  do  with  a  pressure  may 
be  made  to  give  the  required  value  of  g. 

Nevertheless,  the  pendulum  method  remains 
by  far  the  most  precise;  the  value  of  g  which  it 
will  give  in  the  hands  of  a  good  experimenter  may 
attain  a  precision  of  one  part  in  300,000.  It  is  true 
that  many  geodesists  give  values  for  g  as  if  they 
were  precise  to  the  extent  of  one  part,  or  even  less, 
in  1,000,000.  But  this  precision  is  only  apparent 
and  has  its  origin  in  the  figures  resulting  from  the 
application  of  the  method  known  as  that  of  least 
squares,  made  use  of  in  the  discussion  of  experi- 
mental errors.  This  method,  though  excellent 
in  certain  cases,  often  masks  errors  of  experiment. 
If  we  have  twelve  numbers  each  of  seven  figures, 
which  express  twelve  different  measurements  of 
some  one  quantity,  and  if  the  first  five  figures 
are  common  to  all  the  twelve  values,  we  may 
affirm  that  the  experimental  precision  of  the 
measure  is  expressed  by  the  decimal  order  of  the 
last  of  these  figures;  in  the  case  cited,  this  will  be 
a  precision  of  the  order  of  one  part  in  100,000. 
If  the  results  be  discussed  by  means  of  the  mathe- 
matical theory  of  probabilities,  in  particular  by 
the  method  of  least  squares,  a  probable  error  will 
be  found  less  than  a  certain  number  in  taking 
a  value  which  the  calculation  determines.  But 


Gravity  149 

this  is  a  theoretical  precision  and  not  an  experi- 
mental one.  If  the  twelve  numbers  above  taken 
as  an  example  express,  let  us  say,  twelve  determi- 
nations of  the  coefficient  of  expansion,  a  physicist 
who  will  have  to  use  the  value  of  this  coefficient 
in  subsequent  work  should  only  take  the  first  five 
figures  as  exact  ones ;  they  are  the  only  ones  which 
he  can  be  quite  sure  about,  since  they  are  common 
to  all  the  twelve  determinations.  The  use  of  one 
of  the  subsequent  figures  may  lead  to  a  greater 
or  less  degree  of  probability,  but  not  to  certainty. 

We  shall  now  briefly  consider  the  results  that 
the  methods  above  summarised  have  given,  with 
reference  to  the  point  of  view  of  the  variation  of 
the  intensity  of  gravitation  over  the  surface  of 
the  globe. 

First  of  all,  what  is  the  absolute  value  of  gravity? 
This  quantity  has  been  determined  by  General 
Wefforges  at  the  laboratory  of  weights  and  meas- 
ures at  Sevres,  in  1890-2,  and  at  Vienna  by  General 
von  Sterneck.  The  absolute  value  of  gravity  at 
Paris  is  980.97  centimetres  [386.208  in.]  and  at 
Vienna  979.98  centimetres  [385.818  in.].  Some 
explanation  must  be  given  as  to  why  the  values 
should  be  expressed  in  centimetres.  The  reason 
is  the  application  of  a  law  relating  to  uniformly 
accelerated  movement.  It  may  be  shown  by 


150  The  Earth 

mechanics,  that  when  a  constant  force  acts  on  a 
body  originally  at  rest,  it  communicates  a  move- 
ment of  uniform  acceleration  to  the  body,  or  in 
other  words  a  movement  the  velocity  of  which 
increases,  during  each  unit  of  time,  by  the  same 
constant  quantity,  which  is  called  the  acceleration. 
Acceleration  is  therefore  a  length,  and  is  naturally 
expressed  in  centimetres.  The  contradiction  be- 
tween the  expression  of  gravity  by  a  length  and 
what  has  been  shown  above  as  to  its  variability 
will  be  noted.  The  expression  of  the  intensity 
of  a  force  by  the  degree  of  acceleration  which  it 
imparts  to  a  material  mass  implies  that  the  force 
in  question  remains  constant.  Now  gravity  is 
only  a  constant  force  on  condition  that  the  body 
on  which  it  acts  remains  absolutely  immobile; 
if  it  move,  either  up  or  down  or  parallel  to  the 
surface  of  the  globe,  the  value  of  gravity  changes. 
In  order  to  obtain  an  acceleration  to  measure  the 
force  producing  it,  the  body  must  fall  and  hence 
there  will  be  a  variation  in  the  value  of  the  force 
acting  on  it. 

To  avoid  this  difficulty  we  conventionally  define 
the  acceleration  of  gravity,  and  imagine  that 
gravity,  which  has  a  fixed  value  when  it  acts  on  a 
body  placed  at  a  certain  point,  will  maintain  the 
same  value  during  the  fall  of  the  body  on  which  it 


Gravity  151 

acts.  Then,  and  then  only,  will  the  movement  of 
fall  be  uniformly  accelerated  and  such  uniform 
acceleration  may  serve  as  the  measure  of  the 
intensity  of  gravity  at  the  initial  position  of  the 
body.  It  will  be  seen  that  these  imaginary  con- 
ditions are  not  practically  realisable. 

With  this  understanding,  viz.,  the  imaginary 
hypothesis  as  to  the  constancy  of  gravity  through- 
out the  extent  of  fall  of  the  body,  we  shall  now 
consider  how  these  ideas  may  be  applied  to  the 
measurement  of  the  intensity  of  gravity  by  means 
of  the  pendulum. 

A  pendulum  is  a  falling  body;  at  its  position  of 
equilibrium  it  merely  represents  a  plumb  line, 
but  when  pulled  aside  from 
this  position  it  tends  to  re- 
turn to  it,  since  it  is  acted 
on  by  gravity  and  caused  to 
fall  down  again  towards  its 
original  position,  which  is 
that  nearest  the  Earth's 
centre.  The  height  of  the 
fall  is  thus  the  distance  mM  FIG.  12.— Precision  of 
(Fig.  12).  This  distance  de-  Pendulum  Measures- 
fines  the  degree  of  precision  of  pendulum  measures. 
For  a  fall  of  300  metres  [990  ft.],  the  variation  in 
the  value  of  gravity  is  about  TO ,Voo  part  of  its  origi- 


152  The  Earth 

nal  value.  For  a  fall  of  30  metres  [99  ft.]  it  will 
be  100.000  part;  for  one  of  3  metres  [9.9  ft.], 
1,000.000  part,  and  for  a  fall  of  3  centimetres 
[1.17  in.]  it  will  be  one  hundred  millionth,  while 
for  one  of  three-tenths  of  a  millimetre  [.012  in.] 
it  will  be  one  ten  thousand  millionth  part.  Now 
for  an  angle  a  of  oscillation  equal  to  a  degree,  the 
height  mM  is  equal  to  three-twentieths  of  a  milli- 
metre [.005  in.].  Consequently,  the  correspond- 
ing variation  in  the  intensity  of  gravity  is  about 
one  twenty  thousand  millionth  part  of  its  value, 
and  this  fraction  therefore  expresses  the  limiting 
precision  of  pendulum  measurements.  If  it  be 
required  in  the  future  to  attain  a  greater  precision, 
it  will  be  necessary  to  take  account  of  this  little 
variation  in  gravity  during  the  distance  mM  and 
to  devise  a  new  mathematical  analysis  dealing 
with  the  phenomena  in  these  conditions. 

But  the  experimental  precision  of  pendulum 
measures  does  not  actually  exceed  the  500*000 
part  and  perhaps  does  not  attain  even  this,  so 
that  in  practice,  in  the  operation  of  measuring 
the  acceleration  of  gravity  by  the  aid  of  pendulum 
observations,  we  may  neglect  the  almost  imper- 
ceptible variation  in  the  force  along  the  path  of 
the  fall,  the  resulting  error  being  far  smaller 
than  the  errors  of  experiment.  Consequently  the 


Gravity 


153 


actual  methods  used  for  the  determination  of  g 
are  legitimate  and  accurate. 

There  is  a  force  which  partially  opposes  that  of 
gravity,  namely,  the  centrifugal  force  due  to  the 
Earth 's  rotation  and  we  have  to  compound  the 
two  forces  to  find  the  resultant  effect.  A  point  A 


-,-Q 


FIG.  13. — Form  of  the  Earth  determined  by  the  combined 
actions  of  Gravity  and  Centrifugal  Force. 

(Fig.  13)  on  the  surface  of  the  Earth  (supposed 
spherical)  is  attracted  towards  the  centre  by  a 
force  AF  but  at  the  same  time  the  Earth's 
rotation  tends  to  drive  it  in  the  direction  AQ,  per- 
pendicular to  the  polar  axis.  The  point  A  is  con- 
sequently subjected  to  two  forces,  the  attraction 
AF  and  the  centrifugal  force  AQ ;  this  last  increases 
in  proportion  as  the  point  lies  nearer  the  equator 
and  diminishes  with  decreasing  distance  from  the 
pole.  The  resultant  of  these  two  forces,  that  is 


154  The  Earth 

to  say  the  real  force  to  which  the  point  A  is  sub- 
jected, is  thus  a  force  AN  which  is  not  directed 
toward  the  centre  of  the  sphere.  Since  the  Earth 
was  originally  fluid  it  attained  an  equilibrium 
surface  such  that  it  was  perpendicular  at  every 
point  to  the  resultant  force  at  that  point.  It  may 
be  shown  mathematically  that  such  a  surface 
would  be  an  ellipsoid  of  revolution  rotating  about 
its  minor  axis  and  this  is  why  the  Earth  is  not 
spherical  but  has  an  ellipsoidal  figure.  As  a 
result,  the  pole  P  is  nearer  to  the  centre  O  than 
a  point  E  on  the  equator,  the  former  distance  being 
less  and  the  latter  one  greater  than  it  would  be 
if  the  Earth  were  spherical.  In  this  case  the 
equator  would  occupy  the  position  ee.  It  follows 
that  the  Earth  bulges  at  the  equator,  having  the 
added  portion  Ee  formerly  mentioned. 

It  may  be  calculated  that  at  the  equator  the 
centrifugal  force  is  air  part  of  that  of  gravity, 
and  289  being  the  square  of  17  we  have  noted  the 
consequences  that  would  result  if  the  Earth  turned 
seventeen  times  quicker.  Gravity  would  then 
be,  at  the  equator,  exactly  balanced  by  the  centri- 
fugal force,  and  no  body  would  have  any  apparent 
weight.  As  it  is,  this  force  is  sufficient  to  cause 
the  variation  of  gravity  to  the  extent  of  -^  part 
between  the  equator  and  the  pole. 


Gravity  155 

Another  cause  also  produces  a  variation  of 
gravity  between  the  equator  and  the  poles.  Be- 
cause of  the  Earth's  ellipticity,  points  on  the 
equator,  farther  from  the  centre,  are  attracted 
less  than  points  close  to  the  poles  which  are  nearer 
the  centre.  This  second  cause  of  a  continuous 
variation  of  gravity  with  position  on  the  Earth's 
surface  acts  in  the  same  way  as  the  first,  viz.,  a 
decrease  of  gravity  towards  the  equator.  There 
is  also  a  third  cause,  which  on  the  contrary,  acts 
in  the  opposite  way  from  the  first  two,  viz.,  the 
influence  of  the  equatorial  protuberance.  This 
mass,  Ee  in  section,  exercises  an  additional  attrac- 
tion beyond  what  would  occur  in  the  case  of  a 
perfect  sphere,  and  so  increases  the  force  of  gravity 
at  the  equator. 

Geodesists  have  found  a  simple  formula  to 
express  the  resultant  variation  of  gravity  between 
the  Earth's  poles  and  equator,  which  allows  for 
all  three  causes.  If  we  know  the  value  g0  of 
gravity  at  the  equator,  its  value  in  a  place  whose 
latitude  is  X  is  given  by  adding  to  g0  a  fraction  of 
g0  obtained  by  multiplying  it  by  the  yj^  part 
of  the  square  of  the  sine  of  the  latitude.1  Since 
•j-J-3  is  nearly  T^O*  we  can  state  that  in  round 
numbers  a  body  moved  from  the  equator  to  the 

IgA=g0(i+Tksin2X). 


156  The  EartH 

pole  apparently  gains  in  weight  ^  part  of  its 
original  weight.  If  the  body  weighs  a  kilogram 
[2  Ib.  3  oz.  4  dr.]  at  the  equator,  on  a  dyna- 
mometer (not  on  a  balance),  it  will  weigh  5  grams 
[77  gr-l  more  at  the  pole. 

The  value  g0  of  gravity  at  the  equator  is 
expressed  by  978.07  centimetres  [385.057  in.]. 

Pendulum  observations  can  therefore  serve  for 
the  determination  of  the  flattening  of  the  Earth. 
Clairaut  was  the  first  to  give  a  complete  analysis 
of  this  matter  and  it  was  again  undertaken  by  the 
eminent  German  geodesist  Helmert,  who  in  1901 
found  the  value  -^  from  a  discussion  of  all  the 
determinations  of  gravity.  Geodetic  measure- 
ments by  direct  measures  of  meridian  arcs  gave 
Bessel  the  result  -^9,  and  astronomical  calcula- 
tions based  on  precession,  nutation,  and  ine- 
qualities in  the  Moon's  movements  have  led  to 
the  value  -^  deduced  by  the  mathematicians 
Radau,  Poincare,  and  Hill.  The  agreement  be- 
tween the  three  figures  obtained  by  such  different 
methods  is  truly  wonderful. 

To  the  continuous  regular  variations  of  gravity 
must  be  added  that  caused  by  the  elevation  of  the 
point  of  observation  above  the  geoid,  the  cause  of 
which  is  illustrated  by  the  experiment  of  Jolly, 
previously  described;  its  calculation  is  easy.  But 


Gravity  157 

there  are  also  accidental  variations  of  gravity, 
anomalies,  which  we  shall  now  deal  with. 

We  have  seen,  in  the  course  of  the  preceding 
pages,  that  the  measurements  of  the  intensity  of 
gravity  enable  a  precise  value  of  the  flattening  of 
the  terrestrial  globe  to  be  obtained.  We  have 
seen  that  the  laws  of  the  central  attraction  and 
centrifugal  force  combined  would  impose  on  the 
originally  fluid  Earth  the  shape  of  an  ellipsoid  of 
revolution,  turning  about  its  minor  axis,  which 
produces  the  flattening  experimentally  found. 

The  problem  of  finding  the  exact  form  of  the 
Earth  is  one  of  a  very  great  complexity.  If  it 
were  required  to  determine  point  by  point  a  figure 
similar  to  that  of  the  Earth  with  its  contour  pro- 
jections and  representation  of  the  various  alti- 
tudes, the  problem  would  surpass  human  power. 
Fortunately  several  simplifications  are  possible. 
In  order  to  resolve  the  question,  sufficiently,  the 
thing  to  do  is  to  consider  three  surfaces,  each 
defined  in  a  different  way.  These  surfaces  are  as 
follows:  First,  the  geographical  surface,  that  is  to 
say,  the  exterior  surface  of  the  terrestrial  globe, 
comprising  the  surface  of  the  continents  and  the 
free  surface  of  the  seas  and  on  which  rests  the 
atmosphere  that  envelops  us;  secondly,  the  geoid, 
of  which  we  have  already  spoken,  and  which  is  the 


158  The  Earth 

oceanic  surface  supposed  to  be  prolonged  below 
the  continents;  thirdly,  the  geodetic  surface,  which 
is  defined  geometrically  and  which  will  be  a  sur- 
face of  reference  for  the  purpose  of  our  study;  it 
is  an  ellipsoid  of  revolution  calculated  to  agree 
exactly  with  the  most  precise  and  extended 
measurements  of  meridian  arcs. 

The  geographical  or  real  surface  is  that  on  which 
we  live  and  on  which  consequently  all  our  obser- 
vations and  experiments  are  made,  including  the 
astronomical  determination  of  fixed  reference 
points  in  the  celestial  sphere.  On  this  surface  of 
the  crust  we  have  measured  arcs  of  the  meridian 
which  have  enabled  us  to  determine  the  dimen- 
sions of  the  ellipsoid  of  reference.  But  what  we 
wish  to  know  is  the  exact  form  of  the  second  sur- 
face above  mentioned,  viz.,  the  geoid,  the  liquid 
surface  which  by  its  fluidity  obeys  the  combined 
laws  of  attraction  and  centrifugal  force.  More- 
over we  know  that,  on  account  of  the  relatively 
very  slight  mean  continental  altitude,  as  compared 
with  the  Earth's  dimensions,  the  real  surface  does 
not  differ  much  from  the  surface  of  the  geoid. 

We  are,  thus,  lead  to  study  a  surface,  the  geoid, 
that  is  intimately  related  to  the  real  surface,  and 
which  does  not  differ  greatly  from  the  surface  of 
reference,  that  is  to  say,  the  theoretical  ellipsoid. 


Gravity  159 

If  gravity  were  always  the  resultant  of  the 
centrifugal  force  and  of  the  attractive  force,  the 
geoid  would  theoretically  coincide  with  the  theo- 
retical ellipsoid.  But  gravity,  the  direction  of 
which  is  found  at  every  point  by  that  of  a  plumb 
line,  that  is  to  say  by  the  true  vertical,  is  not 
simply  this  resultant  at  every  point ;  it  is  subject 
to  anomalies  arising  from  the  local  irregularities 
of  the  crust,  such  as  high  continental  plateaux, 
mountains,  beds  of  minerals  of  different  densities 
from  the  surrounding  rocks,  the  discontinuities 
between  land  and  sea,  etc.,  which  introduce  local 
attractions  acting  on  and  deviating  the  heavy 
mass  suspended  at  the  end  of  the  plumb  line,  thus 
deflecting  the  vertical. 

And,  as  the  geoid  is  normal  to  the  direction  of 
the  vertical  at  each  point  of  its  surface,  it  follows 
that  every  anomaly  of  gravity,  every  local  devia- 
tion of  the  vertical,  introduces  a  deformation  into 
the  surface  of  the  actual  geoid. 

It  is,  however,  to  be  noted  that  an  analysis  of 
the  matter  will  afford  us  much  useful  information 
with  which  to  begin  our  study. 

In  the  first  place,  it  may  be  shown  that  it  is 
always  possible  to  place  a  given  ellipsoid  of  revolu- 
tion, chosen  as  surface  of  reference,  in  such  a  way 
that  its  surface  shall  contain  a  given  point  of  the 


160  The  Earth 

geoid  and  that  at  this  point  the  astronomical 
longitude,  latitude,  and  azimuth  shall  be  respec- 
tively equal  to  the  geodetic  longitude,  latitude,  and 
azimuth  defined  by  means  of  the  surface  of  refer- 
ence. Also,  when  this  is  done  the  axis  of  revolu- 
tion of  the  ellipsoid  is  parallel  to  the  polar  axis 
of  the  Earth.  Then  in  every  point  where  the  real 
vertical  of  a  place  on  the  geographical  surface 
intersects  the  ellipsoid  of  reference,  we  may  ob- 
tain the  astronomical  elements,  above  mentioned, 
by  observation  and  the  geodetic  elements  by 
calculation. 

We  have  said  that  the  local  attractions  modify, 
in  places,  the  actual  surface  of  the  geoid,  and  in 
such  places  the  geoid  does  not  coincide  with  the 
surface  of  reference.  Consequently  as  the  latter 
is  known  by  definition,  in  order  to  find  the  true 
surface  of  the  actual  geoid,  it  is  necessary  to  know 
the  vertical  distance  separating  the  points  where 
the  normal  to  the  ellipsoid  meets,  in  the  first 
place,  the  ellipsoid,  and  in  the  second  place,  the 
geoid;  this  is  astronomical  levelling.  It  is  thus 
necessary  to  have  the  largest  possible  number  of 
observations  of  precise  astronomical  observations 
made  on  the  real  geographical  surface. 

The  lack  of  homogeneity  in  the  constitution  of 
the  terrestrial  crust,  and  the  separation  of  lands 


Gravity  161 

and  seas  with  the  consequent  discontinuity  of 
density,  are  the  chief  causes  of  local  anomalies 
and  hence  of  the  corresponding  deviations  of  the 
direction  of  the  vertical.  The  pendulum,  with 
its  precision  of  about  I  part  in  500,000,  may  be 
used  to  discover  local  gravitational  anomalies. 
If  gravity  at  a  place  is  not  affected  by  such  causes 
it  has  the  value  which  the  formula  previously  given 
assigns,  depending  on  the  square  of  the  sine  of 
the  latitude.  If  the  pendulum  observation  gives  a 
different  value,  there  is  an  anomaly,  positive  or 
negative  according  as  the  observed  value  of  grav- 
ity is  greater  or  less  than  the  calculated  value. 
The  numerical  value  of  the  difference  between 
observation  and  calculation  gives  the  anomaly, 
qualitatively  and  quantitatively  after  the  value 
measured  has  been  reduced  to  sea-level  by  cor- 
rection, and  after  the  altitude  of  the  place  of 
observation,  which  implies  a  diminution  of  the 
attractive  force  that  is  easily  worked  out,  has 
been  allowed  for. 

The  anomalies  of  gravity  are  of  two  kinds: 
sometimes  they  are  purely  local  and  due  to  the 
attraction  of  neighbouring  masses,  whether  the 
higher  parts  of  the  relief,  such  as  mountains  or 
whether  underneath  the  ground  in  the  form  of 
mineral  deposits  of  abnormal  density.  Sometimes 

XX 


162  The  EartH 

also  they  have  a  systematic  character  and  follow 
a  veritable  law  with  regard  to  their  variations. 

Local  anomalies  have  a  very  great  significance 
for  geologists ;  they  give  valuable  indications  as  to 
the  density  and  position  of  mineral  masses  which 
it  is  impossible  to  see  or  sometimes  even  to  reach 
at  all.  An  earnest  and  profound  study  of  the 
anomalies  has  led  German  geodesists  and  geologists 
to  discover  a  long  subterranean  mass,  directed 
from  west  to  east,  passing  under  the  Elbe  and 
the  Oder.  Swiss  geodesists,  from  anomalies  ob- 
served in  their  country  have  arrived  at  some  very 
remarkable  conclusions  as  to  the  constitution  of 
the  subsoil,  in  particular  in  the  region  of  the 
Engadine. 

Heim  of  Zurich  has  emphasised  the  importance 
of  these  studies,  in  relation  to  the  conditions  of 
formation  of  the  terrestrial  crust.  The  earth  is 
constituted  by  a  lithosphere  (mineral  crust)  and 
a  barysphere  (internal  nucleus  of  great  density). 
On  the  site  of  the  great  original  masses,  such  as 
Mt.  Blanc,  heavier  internal  masses  were  nearer 
the  surface,  and  were  raised  up  on  the  spot.  On 
the  contrary,  where  the  density  of  the  lithosphere 
is  less,  it  has  been  accumulated  by  a  succession 
of  layers  and  has  sunk  down  into  the  fluid  part 
below  in  proportion  to  the  added  burden,  the 


Gravity  163 

upper  regions  of  the  barysphere  being  driven  back 
laterally.  We  should  thus  expect  to  find  the 
vertical  attraction,  that  of  gravity,  stronger  near 
the  great  primary  masses  than  on  the  regions 
formed  by  the  accumulation  of  layers,  and  this  is 
what  observation  confirms.  So,  reciprocally,  the 
study  of  the  values  of  gravity  will  enable  us  to 
distinguish  between  one  or  other  of  these  two 
categories  of  country. 

The  study  of  the  local  anomalies  exhibited  by 
gravity  has  thus  an  enormous  importance  from  the 
point  of  view  of  the  constitution  of  the  subsoil. 
This  subject  has  been  carried  to  a  quite  unexpected 
degree  of  precision  by  the  work  and  methods  of 
the  famous  Hungarian  physicist,  Baron  Roland 
Eotvos,  in  the  course  of  the  last  ten  years.  He 
endeavoured  to  find  what  was  the  form  of  a  sur- 
face normal  in  every  point  to  the  real  directions 
of  the  vertical,  the  minutest  deviations  of  which 
he  has  managed  to  disclose.  His  apparatus  con- 
sists of  a  torsion  balance  with  a  unifilar  suspension 
enclosed  in  a  copper  cage  having  double  walls; 
the  system  has  a  very  considerable  period  of 
oscillation,  which  extends  to  about  twenty  minutes. 
The  oscillations  of  the  horizontal  bar  were  ob- 
served in  two  successive  perpendicular  planes  and 
from  this  Baron  Eotvos  showed  that  it  is  possible 


164  TKe  Earth 

to  calculate  mathematically  the  principal  radii 
of  curvature  of  the  unknown  surface  whose  form  is 
required.  That  being  done,  he  took  a  second 
instrument  in  which  the  equal  weights,  suspended 
from  the  extremities  of  the  oscillating  lever,  are 
no  longer  in  the  same  horizontal  plane,  but  are 
at  different  levels,  one  of  the  weights  hanging 
below  the  lever,  on  a  thread.  This  arrangement 
enables  the  variations  of  the  force  of  gravity 
along  the  unknown  surface  to  be  measured. 

By  the  aid  of  these  torsion  balances,  Baron 
Eotvos  succeeded  in  showing  that  the  mountainous 
slopes  of  Buda  were  prolonged  under  the  soil  of 
the  town  of  Pest,  for  which  purpose  the  pendulum 
lacked  sufficient  sensitiveness  and  precision.  He 
has  also  been  able  to  prove  variations  of  level  in 
the  Hungarian  rivers  and  lakes;  in  the  case  of 
the  Danube  he  was  able  to  demonstrate  such 
variations  about  100  metres  from  the  bank. 

As  to  the  systematic  anomalies,  they  seem  to 
follow  laws  which  may  be  elucidated  by  dividing 
the  observation  stations  into  three  categories. 
First,  continental  stations,  situated  in  the  midst 
of  continents;  secondly,  insular  stations,  situated 
upon  islands,  in  the  middle  of  oceans,  and,  thirdly, 
coast  stations,  lying  near  the  line  of  separation  of 
a  continent  and  a  sea.  * 


Gravity  165 

Generally  speaking,  continental  stations,  for 
example  those  situated  on  the  Thibetan  plateau, 
in  the  centre  of  Asia,  show  a  deficit  of  gravity  as 
compared  with  the  theoretical  value.  Conversely, 
insular  stations  surrounded  by  a  mass  of  water 
of  less  density  exhibit  an  excess  of  gravity  over 
the  calculated  value.  At  the  Sandwich  Islands, 
to  take  a  particular  example,  the  excess  differ- 
ence reaches  2-oVo  part  of  the  theoretical  value  of 
gravity,  and  the  extraordinary  magnitude  of  this 
irregularity  has  given  rise  to  numerous  hypotheses. 
One  of  these  supposes  that,  taking  into  considera- 
tion the  immense  extent  of  the  Pacific  Ocean,  its 
waters  are  banked  up  on  its  two  shores  by  the 
attraction  exercised  upon  them  by  the  Asiatic 
and  American  continents,  so  that  in  the  central 
region  there  would  be  a  compensating  lowering 
of  the  sea-level.  This  would  explain  the  anomaly, 
because  the  sea-level  at  the  Sandwich  Islands 
would  be  nearer  to  the  Earth's  centre  than  would 
be  the  case  if  no  such  lowering  existed.  This 
hypothesis  is  ingenious,  but  unfortunately  if  the 
experimental  data  be  worked  out  it  is  found  that 
in  order  to  produce  the  above-mentioned  difference 
of  foVo  part  of  the  entire  value,  by  approach- 
ing nearer  the  Earth's  centre,  a  lowering  of  the 
level  of  the  Pacific  by  more  than  1200  metres 


1 66  TKe  Earth 

[%  of  a  mile]  would  be  required.  We  are  not  able 
by  any  measurement  to  prove  this  lowering  of  the 
level  of  the  open  ocean. 

As  to  the  shore  stations,  there  is  usually  an 
excess,  though  on  some  shores  a  deficit.  It  is  a 
remarkable  fact  that  along  any  one  coast  the 
difference  between  observed  and  calculated  values 
has  always  the  same  sign. 

It  has  for  a  long  time  been  believed  that  the 
excessive  attracting  force  experienced  on  oceanic 
islands  is  only  a  particular  case  of  a  general  excess 
of  gravity  existing  at  the  surface  of  the  oceans. 
Dr.  Hecker  of  Potsdam  has  attempted  gravity 
determinations  at  sea  by  the  hypsometer  method. 
We  have  seen  that  the  precision  of  this  hypso- 
barometric  method  does  not  exceed  40.000  or 
60*000  part.  Dr.  Hecker  has  shown  that,  to  this 
degree  of  approximation,  the  force  of  gravity  is 
sensibly  normal  in  the  Atlantic  between  Lisbon 
and  Bahia.  On  the  other  hand,  he  has  proved 
large  local  anomalies  on  the  Pacific,  reaching  as 
much  as  6~oVo  and  even  soVo  part,  some  positive 
(south  of  Australia  and  in  the  Honolulu  roads) 
and  others  negative  (above  the  depths  round 
the  Tonga  Isles,  which  surpass  9000  metres 
[5.6  miles]).  Over  the  rest  of  the  Pacific  Ocean, 
gravity  seems  to  be  normal.  Nevertheless  these 


Gravity  167 

results  indicate  serious  anomalies  in  the  distribu- 
tion of  the  intensity  of  gravity  in  the  Pacific. 

The  real  form  of  the  geoid  will  not  be  fully 
known  until  we  can  measure  gravity  at  sea  with 
as  great  a  precision  as  can  now  be  attained  on 
land,  since  the  land  surface  covers  scarcely  a 
quarter  of  the  surface  of  the  globe  and  the  form 
of  the  geoid  is,  therefore,  unknown  for  three- 
fourths  of  its  extent. 

If  continuous  and  precise  experiments  should 
enable  rigorous  measurements  of  gravity  to  be 
made  on  the  oceans,  and  if  these  showed  that  the 
excess  values  exhibited  on  the  oceanic  isles  were 
a  general  fact  and  applied  to  the  entire  oceans, 
and  furthermore  if  a  similar  certainty  could  be 
arrived  at  with  regard  to  regions  in  the  midst 
of  the  great  continents,  such  as  the  Asiatic  plateau, 
viz.,  that  the  value  of  gravity  is  always  in  deficit 
there,  the  beautiful  hypothesis  of  Lippmann, 
relating  to  the  constitution  of  the  terrestrial 
crust,  would  be  of  great  importance. 

We  have  already  said  a  few  words  about  this 
hypothesis;  at  the  commencement  of  the  solidi- 
fication of  the  crust,  the  first  solid  scoriae  floated 
on  the  surface  of  the  still  liquid  spheroidal  bath, 
each  one  being  sustained  by  the  Archimedean 
thrust  exerted  on  it  by  the  liquid.  In  the  case 


1 68  THe  Earth 

of  those  pieces  surmounted  by  mountains  or 
heavy  continental  masses  the  weight,  being  rela- 
tively more  considerable,  would  cause  them  to  sink 
lower  and  they  would  accordingly  plunge  more 
deeply  into  the  mass  of  liquid  material  on  which 
they  floated.  On  the  other  hand,  those  which 
carried  an  ocean,  of  relatively  slight  weight,  would 
not  sink  in  so  deeply.  In  other  words,  the  ter- 
restrial crust  ought  to  be  thicker  under  the  conti- 
nents than  under  the  oceans. 

The  general  anomalies  of  gravity  would  thus 
be  explained;  under  the  continents  the  greater 
thickness  of  the  crust,  increased  by  the  height 
of  the  superincumbent  mass,  would  render  the 
distance  from  the  surface  of  the  soil  to  that  of 
the  barysphere  greater,  and  it  is  the  barysphere, 
the  nucleus  of  very  great  density,  which  gives  rise 
to  the  greater  part  of  the  attracting  force.  This, 
being  inversely  as  the  square  of  the  distance, 
would  consequently  be  reduced  for  points  of  the 
continental  surface.  Also  this  diminution  is  not 
compensated  by  the  additional  attraction  exerted 
by  the  continental  mass  itself,  for  that  has  only 
the  density  of  its  constituent  rocks,  2.5  times 
that  of  water,  while,  as  we  have  seen,  the  density 
of  the  central  nucleus,  the  barysphere,  is  much 
higher. 


Gravity  169 

As  regards  the  oceans,  on  the  contrary,  the 
relative  thinness  of  the  crust  results  in  the  bary- 
sphere  being  much  nearer  to  the  geographical 
surface  and  consequently  exerting,  at  that  surface, 
an  attractive  force  of  greater  intensity.  In  this 
way,  the  excess  of  oceanic  gravity,  always  assum- 
ing it  is  proved,  would  be  readily  explained.  Thus, 
the  future  of  the  study  of  gravity  lies  on  the  sea 
as  also  does  that  of  other  sciences,  meteorology 
in  particular.  This  should  not  be  surprising, 
considering  the  extent  of  the  water  superficies 
and  its  uniform,  homogeneous  and  regular  surface. 
It  is  natural  that  the  general  laws  to  which  the 
Earth  is  subject  apply  fundamentally  to  these 
great  liquid  areas. 

In  any  case,  the  real  geoid,  after  all  we  have 
just  said,  is  not  an  exact  ellipsoid,  but  one  modified 
in  places  by  local  anomalies  which  produce  pro- 
tuberances or  hollows  as  the  case  may  be ;  the  dif- 
ferences between  the  two  surfaces  are  not  very 
considerable,  since  the  German  geodesist  Helmert 
has  shown  that  the  real  geoid  never  deviates  more 
than  200  metres  [656  ft.]  from  the  theoretical 
ellipsoid. 

We  have  seen,  in  the  course  of  the  pages  of  this 
chapter,  how  the  Earth's  attraction  produces  at 
the  surface  the  continual  phenomenon  of  the  fall 


170  The  Earth 

of  bodies  towards  the  centre.  Each  such  fall 
has  an  effect  on  the  intensity  of  gravity  at  the 
surface  of  the  globe.  When  I  raise  a  weight  a 
small  height  in  my  laboratory  in  Paris  and 
let  it  fall,  I  change  at  the  same  moment  the 
intensity  of  gravity  at  Honolulu,  and,  in  gen- 
eral, over  all  the  Earth's  surface,  for  in  allowing 
the  body  to  drop  to  the  ground  I  add  the  at- 
tracting mass  of  this  body  to  that  of  the  terrest- 
rial spheroid. 

Another  cause  of  the  variability  of  gravity  is 
thus  adduced.  It  is  true  that  the  variation 
from  this  cause  is  infinitesimal,  and  so  cannot  be 
experimentally  shown,  but  nevertheless  it  is  real. 
The  variability  of  gravity  from  other  reasons  is 
sufficient  to  confirm  the  idea  of  the  law  of  in- 
stability, change,  and  general  evolution  which 
governs  the  existence  of  the  planet  on  which  we 
live. 

There  is  still  another  cause  of  variation  of 
gravity,  which  produces  a  secular  variation.  At 
the  present  time  this  variation  is  insensible, 
though  it  must  have  been  large  on  taking  into 
consideration  the  change  in  our  globe  since  the 
time  of  its  formation.  The  cause  here  referred 
to  is  the  contraction  of  the  globe  due  to  its  cooling 
process. 


Gravity  171 

The  force  of  gravity  acting  on  a  body  placed  on 
the  surface  of  the  Earth  is  in  inverse  proportion 
to  the  square  of  its  distance  from  the  Earth's 
centre,  that  is  to  say,  in  general,  to  the  square  of 
the  terrestrial  radius.  If,  therefore,  the  radius 
decrease  owing  to  cooling,  the  force  of  gravity 
will  increase  accordingly. 

A  contraction  of  one-fifth  part  of  the  radius 
would  produce  an  increase  in  gravity  of  ^  of  the 
value  of  the  latter,  in  other  words  a  little  more 
than  50%  of  the  value. 

The  hypothesis  of  a  shortening  of  the  radius 
of  our  globe  agrees  well  with  the  fact  of  the  fold- 
ings of  its  crust.  Gravity  at  the  Earth's  surface 
has  consequently  increased.  The  importance  of 
this  fact  is  considerable  in  connection  with  the 
pressure  of  the  atmosphere  which,  formerly,  must 
have  been  much  less  than  at  present,  in  the  same 
conditions  of  temperature.  Even  the  composi- 
tion of  the  atmosphere  must  have  changed  from 
this  cause  and  would  vary  in  proportion  to  the 
contraction  of  the  Earth  as  cooling  unceasingly 
continued. 

Since  this  cooling,  though  now  quite  insensible, 
yet  goes  on,  the  slow  contraction  is  still  occurring 
and  the  force  of  gravity  at  the  surface  of  the  globe 
consequently  increasing. 


172  The  Earth 

To  conclude,  this  force,  which  was  formerly 
stated  to  be  constant  in  magnitude  and  direction, 
is,  on  the  contrary,  subject  to  continuous  variation, 
both  as  regards  time  and  space. 


CHAPTER  VI 


THE  RHYTHMIC  MOVEMENTS  OF  THE  EARTH'S  CRUST. 
DEVIATIONS  OF  THE  VERTICAL 


TN  our  study  of  the  Earth's  movements,  we  have 
*  seen  how  great  is  the  complexity  of  the  motion 
of  any  given  point  upon  its  surface;  we  have  also 
demonstrated  and  sought  to  explain  the  continu- 
ous displacement  of  the  Earth's  poles.  But,  in 
all  this  account,  the  Earth  has  been  assumed  to 
be  rigid,  and  to  retain  always  the  flattened  ellip- 
soidal form  imposed  upon  it  by  the  combined 
laws  of  universal  attraction  and  centrifugal  force. 

But  perhaps  the  Earth  is  not  really  rigid.  The 
crust  may  not  be  truly  undeformable.  It  is 
possible  that  it  suffers  deformation  under  certain 
influences.  We  have  to  inquire  whether  this  is 
so  and  under  what  conditions  such  non-rigidity 
becomes  manifest,  also  its  extent  and  importance. 

In  the  course  of  this  chapter,  we  shall  find  that 
the  crust  is  in  a  state  of  perpetual  movement  and 
incessant  vicissitude  which  still  further  emphasises 
the  life,  figuratively  speaking,  of  the  Earth. 

173 


174  The  Earth 

The  illustrious  English  physicist  Lord  Kelvin 
was  the  first  to  suggest  the  question  whether  the 
Earth  is  an  undeformable  solid,  or  if,  on  the  con- 
trary, it  is  an  elastic  body  whose  form  is  incessantly 
modified  by  exterior  causes,  in  particular  by  the 
combined  periodical  and  variable  attractions  of  the 
Moon  and  the  Sun.  The  problem  resolves  itself 
into  finding  proof  of  the  elasticity  of  the  terrest- 
rial crust  and  the  measurement  of  such  elasticity. 

Any  body  whatever,  that  is  free  to  move  at 
the  Earth's  surface,  for  example,  the  heavy  ball 
of  a  plumb  line,  is  always  subjected  to  the  attrac- 
tive forces  exercised  upon  it  by  the  Moon  and  the 
Sun.  The  prolongation  of  the  plumb  line  should 
therefore  describe  some  kind  of  curve  on  the 
ground  beneath  it.  If  the  Earth  were  rigorously 
rigid  and  undeformable,  it  would  not  change  its 
form  under  the  action  of  these  attractive  forces, 
the  only  effect  of  which  is  to  impress  upon  the 
Earth  the  movements  of  rotation,  revolution, 
precession,  nutation,  etc.,  which  have  been  de- 
scribed in  detail  in  Chapter  IV. 

Assuming  the  Earth  to  be  strictly  rigid,  what 
would  be  the  value  of  this  luni-solar  attraction? 
At  first  sight,  it  would  seem  a  large  one.  The 
Sun  has  a  mass  about  325,000  times  greater  than 
the  Earth  and  is  at  a  distance  from  the  latter 


Movements  of  tHe  EartH's  Crust   175 

equal  to  23,400  terrestrial  radii.  If  we  evaluate 
the  attractive  force  strictly  according  to  Newton 's 
law,  viz.,  as  proportional  to  the  product  of  the 
masses  and  in  inverse  ratio  to  the  square  of  the 
distances,  a  result  is  obtained  which  is  about  1300 
times  less  than  that  of  gravity  [at  the  Earth's 
surface. — Ed.].  Consequently  the  luni-solar  at- 
traction is  sufficient  to  produce  an  apparent 
diminution  of  the  weight  of  bodies  here  equal  to 
nVo  part  of  their  real  weight. 

But  it  must  not  be  forgotten  that  the  Earth, 
under  the  action  of  the  solar  attraction,  executes 
its  orbital  movement.  Now  it  is  a  fundamental 
principle  in  mechanics  that  a  force  already  obeyed 
does  not  enter  into  play  except  as  regards  the  effect 
already  produced.  A  heavy  body,  suspended  at 
the  Earth's  surface,  and  which  the  solar  attraction 
tends  to  draw  aside  from  the  vertical,  is  already 
moving  with  the  whole  Earth  under  the  influence 
of  that  attraction.  There  would  consequently 
only  remain,  as  an  effective  deviating  force,  the 
difference  between  the  attractive  force  at  the  sur- 
face and  that  at  the  centre  of  the  Earth.  The 
result  so  obtained  is,  for  the  Sun,  nearly  20,000 
times  less  than  that  above  given  and  is  equivalent 
to  only  the  2«.oo1o.ooo  part  of  the  force  of  gravity 
[at  the  Earth's  surface.— Ed.]. 


176  The  Earth 

The  small  mass  of  the  Moon  is  largely  compen- 
sated, from  the  point  of  view  of  the  extent  of 
attractive  force  it  exercises  on  a  body  placed  at 
the  surface  of  our  globe,  by  its  much  greater  prox- 
imity; the  Moon's  centre  is  only  distant  sixty 
terrestrial  radii  from  that  of  the  Earth.  On 
applying  to  our  satellite  the  same  reasoning  and 
the  same  calculation  that  we  have  already  done 
in  the  case  of  the  Sun,  we  obtain  the  result  that 
the  perturbing  effect  of  the  lunar  attraction  pro- 
duces a  diminution  of  gravity  [at  the  Earth's 
surface. — Ed.]  of  about  i2,oio.ooo  part:  As  the 
arc  corresponding  to  an  angle  of  a  second  is 
about  200^000  it  is  evident  that  the  deviation 
from  the  vertical  due  to  the  influence  of  the  Moon 
attains  about  A  of  a  second. 

We  are  indebted  to  Victor  Puiseux  for  the 
complete  analysis  of  this  perturbing  action.  At 
a  later  date  Gaillot  put  it  into  a  simplified  form 
and  Radau  made  a  more  elementary  calculation 
for  the  case  where  the  Moon  is  in  the  plane  of  the 
equator.  The  astronomer  Gaillot  has  traced  the 
theoretical  curves  which  the  prolongation  of  a 
plumb  line  should  describe  on  a  horizontal  sheet, 
under  the  influence  of  the  lunar  attraction,  the 
Earth  being  assumed  absolutely  rigid.  These 
curves  are  shown  in  the  accompanying  diagrams 


Movements  of  tHe  EartH's  Crxist     177 


(Figs.  14, 15, 1 6, 17).  It  will  be  observed  that  they 
differ  in  accordance  with  the  Moon's  declination, 
or  in  other  words,  with 
its  angular  distance 
from  the  equator. 

When  these  results 
were  known,  and  the 
minuteness  of  the 
quantity  to  be  meas- 
ured, in  order  to  prove 
the  diurnal  variations 
of  the  vertical,  realised, 
many  physicists  gave 
up  hope  of  achieving 
it.  But  others  at- 
tempted to  overcome  FlGS  ^  ^  I6>  I7._curves 

the    difficulties   of   the      theoretically  described    by   the 
T        0  Bob  of  a  Plumb-line,  varying  in 

experiment.  In  1873,  Accordance  with  the  Declination 
Zollner  tried,  for  the  of  the  Moon, 
first  time,  a  horizontal  pendulum,  to  which  we 
will  return  later  in  fuller  detail,  and  which  had 
extreme  sensitiveness;  in  1874,  Bouquet  de  la 
Grye  used  a  pendulum,  connected  with  an  ampli- 
fying balance,  at  Campbell  Isle  where  he  had  gone 
to  observe  the  transit  of  Venus;  and  in  1878, 
Lord  Kelvin  made  use  of  a  long  pendulum  the 
deviations  of  which  were  multiplied  by  means 


178  The  Earth 

of  SL  small  rotating  mirror.  In  1879,  G.  and  H. 
Darwin  perfected  this  apparatus  by  immersing  it 
in  a  liquid  bath  to  preserve  it  from  disturbing 
effects;  in  1881,  d'Abbadie  installed  in  his  obser- 
vatory at  Hendaye,  an  auto-collimating  telescope 
directed  perpendicularly  downward  on  a  bath  of 
mercury  placed  at  the  bottom  of  a  deep  shaft; 
the  variations  from  coincidence  of  a  reticle  at  the 
focus  and  its  reflected  image  should  be  double  the 
variations  of  the  vertical.  In  1883,  Professor 
C.  Wolf,  of  the  Sorbonne,  set  up  an  analogous 
apparatus,  but  a  horizontal  one,  in  the  vaults  of 
the  Paris  Observatory;  and  finally,  in  1890,  in 
the  same  place  the  mining  engineer  Leon  and  I 
attempted  to  arrange  a  very  sensitive  instrument, 
with  communicating  liquid  baths,  the  differences 
of  level  of  which  were  observed  by  interference 
fringes  in  yellow  light.  In  spite  of  the  extreme 
precision  of  the  method  employed,  our  apparatus 
gave  no  more  clearly  affirmative  results  than  those 
of  our  predecessors. 

The  phenomenon  to  be  measured  is  extremely 
small.  A  pendulum  100  metres  [330  ft.]  long 
would  be  very  difficult  to  make,  and  especially 
to  set  up  and  maintain  in  the  necessary  conditions 
of  stability  and  freedom  from  disturbing  effects. 
Yet  even  with  such  a  pendulum  the  deviation  in 


Movements  of  tHe  EartH's  Crust     179 

question  would  be  only  about  TOT  of  a  millimetre 
[io.3ooo  in.]! 

There  is,  however,  another  cause  for  the  lack  of 
success,  and  this  is  to  be  sought  for  in  the  elasticity 
of  the  Earth. 

The  mathematical  considerations  which  serve 
as  the  base  of  the  preceding  experiments  all  depend 
upon  the  hypothesis  that  the  earth  is  rigid  and 
undef ormable ;  if  the  Earth  has  sufficient  elasticity 
to  be  susceptible  to  deformation  under  the  influence 
of  luni-solar  section,  the  whole  is  changed.  The 
entire  Earth  will  then  behave  similarly  to  what 
we  already  know  occurs  in  the  case  of  the  free 
surface  of  the  oceans,  in  other  words  the  litho- 
sphere  or  solid  part  will  exhibit  the  phenomenon 
of  tides  just  as  the  seas  do  under  the  influence  of 
the  same  forces. 

These  deformations,  which  the  solid  part  of 
the  globe  suffer,  are  of  two  quite  distinct  kinds; 
one  only  affects  the  superficial  layers  of  the  crust 
while  the  other  acts  on  the  whole  body  of  the 
Earth.  The  first  is  characterised  by  an  apparent 
deviation  of  the  vertical  with  respect  to  the 
ground;  in  reality,  as  the  deformations  affect  the 
superficial  layers  of  the  Earth,  it  is  the  ground 
which  suffers  displacement  relatively  to  the  ver- 
tical, which  remains  fixed.  Consequently  the 


l8o  The  Earth 

deviations  are  only  apparent.  The  principal 
cause  of  these  apparent  deviations  is  to  be  sought 
in  the  heating  of  the  surface  layers  of  the  Earth 
by  the  solar  rays.  These  rays  warm  the  terrestrial 
globe  just  as  the  spirit  lamp  heats  the  copper 
ball  in  the  classical  experiment  of  Gravesand's 
ring.  Since  the  surface  rocks  and  layers  have  but 
slight  conductivity  for  heat,  only  that  part  of  the 
Earth  turned  towards  the  Sun  is  affected  by  the 
heating  action  and  so  only  this  part  is  expanded 
and  hence  deformed ;  the  antipodes  of  these  regions 
are  not  reached  by  the  solar  warmth  until  twelve 
hours  later.  For  the  same  reason,  viz.,  the  feeble 
thermal  conductivity  of  the  soil,  the  movements 
of  deformation  thus  produced  are  transmitted 
with  difficulty  in  a  downward  direction  and  their 
amplitudes  decrease  very  rapidly  as  we  penetrate 
below  the  surface  of  the  ground.  In  the  Astro- 
physical  Institute  of  Potsdam,  under  the  direction 
of  Professor  Helmert,  the  apparent  oscillation  of 
the  vertical  has  been  found  to  have,  at  the  bottom 
of  a  shaft  25  metres  [83  ft.]  in  depth,  only  J  of 
its  extent  at  the  surface  level  of  the  ground. 

The  heating  caused  by  the  solar  rays  being  the 
principal  cause  of  these  superficial  deformations, 
which  produce  an  apparent  oscillation  of  the 
direction  of  the  vertical,  it  follows  that  such  oscil- 


Movements  of  the  Earth's  Crust     181 

lations  should  have  an  essentially  diurnal  period; 
furthermore  there  should  be  another  period,  an 
annual  one,  due  to  the  greater  or  less  obliquity 
of  the  solar  rays,  caused  by  the  variation  in  the 
Sun's  declination  according  as  it  is  above  or  below 
the  celestial  equator.  This  period  is  superposed 
on  the  first  one  and  the  actual  resulting  period  is 
a  combination  of  the  two. 

The  second  kind  of  deviation  of  the  vertical  is 
a  true  one  and  not  only  an  apparent  one ;  its  cause 
is  to  be  sought  in  the  attractive  forces  exerted  by 
the  Sun  and  the  Moon  on  the  matter  which 
constitutes  our  Earth. 

If  the  Earth  were  perfectly  rigid,  absolutely 
undeformable,  and  totally  devoid  of  elasticity, 
the  luni-solar  attraction  could  not  produce  any 
possible  deformation  of  it,  and  in  this  case  the 
oscillations  of  the  vertical  under  the  influence  of 
these  forces  could  be  calculated  as  previously 
explained.  If  the  Earth,  in  its  entirety,  were 
perfectly  fluid,  that  is  to  say,  if  it  behaved  as  a 
perfect,  and  not  a  viscous,  liquid,  the  exterior 
surface  would  have  a  regular  form,  which  would 
change  continually  under  the  influence  of  the 
luni-solar  attraction.  In  these  circumstances  it 
would  be  impossible  to  prove  the  slightest  change 
in  the  vertical,  since  by  definition  the  terrestrial 


182  The  Earth 

surface,  the  fundamental  surface,  of  elevation, 
would  always  be  normal  to  the  direction  of  the 
plumb  line.  Consequently  the  terrestrial  tides, 
the  deformations,  while  attaining  the  greatest 
amplitude  theoretically  possible,  would  not  be 
demonstrable,  for  lack  of  reference  points,  just 
in  the  same  way  as  the  oceanic  tide  cannot  be 
appreciated  by  a  navigator  in  the  open  sea,  out 
of  sight  of  land,  the  sea  being  assumed  to  be  too 
deep  for  precise  soundings,  which  would  otherwise 
prove  differences  in  the  depth  of  the  water,  to  be 
taken. 

But  in  reality  the  terrestrial  globe  is  very  far 
from  being  a  perfect  fluid.  Without  being  ab- 
solutely rigid,  it  has  a  considerable  degree  of 
hardness.  The  molten  material  constituting  the  in- 
ternal magma  is  subjected  to  such  pressures  that 
the  state  it  exists  in  is  hardly  conceivable  to  the 
mind,  which  in  the  attempt  to  realise  it  is  obliged 
to  picture  conditions  of  which  it  has  had  no  practi- 
cal experience.  Nevertheless  by  a  rigorous  ana- 
lysis of  the  question  based  on  the  known  values  of 
the  precession  of  the  equinoxes  and  of  nutation, 
Lord  Kelvin  has  found  that  the  Earth,  taken  as  a 
whole,  has  a  rigidity  sensibly  equal  to  that  of  steel. 
This  result  is  by  no  means  incompatible  with  the 
state  of  fusion  of  the  metals  constituting  the  cen- 


Movements  of  tHe  EartH's  Crust     183 

tral  nucleus,  since  this  state  is  largely  counteracted 
by  the  formidable  pressures  to  which  they  are 
subjected.  We  may  therefore  admit  that  the 
terrestrial  globe,  taken  in  its  entirety,  possesses 
a  certain  elasticity. 

Owing  to  the  facts  of  this  elasticity  and  the 
luni-solar  attraction,  the  form  of  the  globe  will 
be  modified.  At  the  same  time,  the  action  of  the 
igneous  matter  will  deform  the  superficial  layers, 
and  the  deviation  which  may  be  shown  relatively 
to  the  direction  of  a  plumb  line  will  therefore  be 
only  that  due  to  the  difference  of  these  two  effects. 

This  explains  the  lack  of  success  of  the  experi- 
ments described  above;  all  were  made  on  the  as- 
sumption of  the  absolute  rigidity  of  the  Earth 
with  the  object  of  verifying  the  extremely  slight 
variations  of  the  vertical,  which  were  of  theoretical 
interest.  The  non-rigidity  of  the  ground  dimin- 
ishes these  minute  variations  still  further,  hence 
the  failure  of  methods  and  apparatus  to  show  it 
that  were  hardly  sensitive  enough  even  if  the 
Earth  had  been  quite  rigid. 

The  credit  of  having  demonstrated  these  devi- 
ations, not  only  qualitatively  but  even  quanti- 
tatively, falls  to  Dr.  Hecker,  of  the  Geodetic 
Institute  of  Potsdam.  For  this  purpose  he  util- 
ised the  wonderful  sensitiveness  of  the  horizontal 


184 


The  Earth 


pendulum,    an    instrument    constructed    several 
years   before   by   von   Rebeur-Paschwitz,    which 


reached  a  high 
was  still  further 
The  horizontal 
instrument 


It   is  com- 
zontal   rod 


FIG. 

Horizontal  Pendulum. 


degree  of  perfection,  but 
improved  by  Dr.  Hecker. 
pendulum  (Fig.  18)  is  an 
of  extreme  sensitiveness, 
posed  essentially  of  a  hori- 
T,  fixed  by  two  vertical 
threads  F  and  F'  to  a 
strong  support  S;  the 
points  of  attachment  A 
and  A'  are  not  exactly  one 
1 8.— Principle  of  a  above  the  other,  but  they 
may  be  made  as  nearly  so 
as  is  desired.  A  mass  M  is  fixed  at  the  end  of  the 
lever  T.  In  these  circumstances  the  pendulum 
takes  a  position  of  equilibrium  for  a  given  direction 
of  the  vertical,  but,  if  this  latter  should  change, 
the  pendulum  begins  to  oscillate  with  a  period  the 
same  as  that  which  a  simple  pendulum  would  have 
if  of  length  equal  to  the  distance  between  the 
mass  M  and  the  point  V  where  the  vertical  M  V 
intersects  the  straight  line  joining  the  points  of 
attachment  A  A'  of  the  two  threads.  It  may  be 
seen  by  an  inspection  of  the  figure  that  we  are 
able  to  make  the  length  M  V  as  great  as  is  desired ; 
all  that  is  necessary  is  to  place  the  points  A  and  A' 


Movements  of  the  EartH's  Crust     185 

more  nearly  above  one  another.  We  thus  have 
a  horizontal  pendulum  H  M  which  oscillates  with 
the  same  period  as  a  vertical  pendulum  of  very 
great  length  V  M  N  and  we  may  make  the  length  of 
the  equivalent  simple  pendulum  so  great  that  its 
oscillation  may  show  the  little  displacements 
from  the  vertical  which  we  have  previously 
described. 

Dr.  Hecker  took  two  of  these  pendulums,  the 
shafts  of  which  were  perpendicular  to  one  another; 
their  lengths  and  the  relative  positions  of  the 
points  of  attachment  had  been  regulated  so  that 
they  corresponded  respectively  to  simple  pendu- 
lums of  175  and  117  metres  [574  and  384  ft.]  in 
length.  The  shafts  were  orientated  symmetrically 
with  regard  to  the  meridian  of  the  place.  Two 
mirrors  fixed  on  the  shafts  enabled  the  period  of 
the  oscillations  to  be  registered  photographically 
on  films,  the  distance  of  which  further  increased 
the  amplitude  of  the  deviations,  and  this  was 
already  doubled  by  the  reflection  from  the  mirror. 

By  taking  the  photograms  thus  obtained  and 
constructing  graphs  from  them  by  points,  having 
for  abscissae  and  for  ordinates  the  results  deduced 
from  the  movements  of  the  two  pendulums,  a 
curve  results  which  illustrates  the  displacement  of 
the  point  of  a  plumb-line,  that  is  to  say  of  the 


1 86  THe  EartK 

deviations  of  the  vertical;  such  a  curve  has  been 
constructed  for  every  day  and  the  results  have 
been  collected  in  groups  of  ninety  days  to  furnish 
three-monthly  averages. 

A  diurnal  oscillation  of  the  vertical  to  the  extent 
of  two-thousandths  of  a  second  of  arc  in  the  direc- 
tion of  the  meridian  has  thus  been  distinguished; 
furthermore  the  three-monthly  averages  have 
shown  that  the  amplitude  of  the  oscillation  is  only 
half  in  winter  what  it  is  in  summer.  As  Lallemand 
has  justly  observed,  this  is  a  clear  indication  of  a 
thermal  effect  produced  by  the  heating  of  the 
peripheral  layers  of  the  Earth's  surface  under 
the  action  of  the  solar  rays  and  "these  effects 
overlie  those  of  the  attraction  of  the  Sun  on  the 
pendulums  and  almost  mask  them  entirely." 

Dr.  Hecker  has,  however,  been  able  to  demon- 
strate the  latter  because  of  the  fact  that  the  period 
of  the  thermal  effect  is  twenty-four  hours,  that 
is  to  say,  is  diurnal,  while  for  the  purely  attractive 
solar  action  the  period  is  twelve  hours,  that  is 
to  say,  semi-diurnal.  The  attraction  is  exercised 
similarly  whether  the  point  in  question  is  directly 
opposite  the  Sun  on  the  near  or  far  side  of  the 
Earth  and  therefore  makes  itself  evident  twice  in 
every  twenty-four  hours.  By  combining  the  values 
of  the  deviations  for  corresponding  hours  of  the 


Movements  of  tKe  EartH's  Crust     187 

two  unsymmetrical  periods  of  twelve  hours  each, 
and  taking  the  semi-sum  and  the  semi-difference 
of  the  deviations  for  each  pair,  the  result  sought 
is  obtained,  for  in  the  semi-sum  the  thermal  effect 
is  naturally  eliminated,  since  it  is  equal  and  of 
contrary  sign  in  the  two  terms,  while  the  attractive 
effect  is  not  so  counterbalanced.  It  is  the  con- 
trary as  regards  the  semi-difference,  which  isolates 
the  thermal  effect  by  eliminating  the  attractive 
one. 

In  the  case  of  the  lunar  action,  the  separation 
of  the  two  effects  is  more  easily  made  on  account 
of  the  difference  of  the  periods  of  the  solar  day 
and  the  lunar  day.  Dr.  Hecker  has  been  able  to 
construct  the  experimental  curve  which  the  point 
of  a  plumb-line  describes  under  the  Moon's  influ- 
ence, by  a  graphical  interpretation  of  his  obser- 
vational results. 

This  curve  is  shown  by  a  dot-       /'  *"\ 

ted  line  (Fig.  19).    Fig.  2Orepre-       \. 
sents  the  same  curve  in  the  case         .... 
where    the    declination    of    the     / 


Moon  is  a  high  northern  one.      v- "*"* 

11               r   , -•  FIGS.  19  and  20. — 

The  exact  resemblance  of  these  ReaiCurvesde- 

experimental    curves    with    the  scribed  by  the  Bob 

1      /-»    -11    .  °f a  Plumb-line, 
theoretical  ones  given  by  Gaillot, 

and   reproduced   in    Figs.  14   and    1 6,    is  very 


1 88  The  EartH 

striking.  The  only  difference  is  in  the  lesser 
amplitude  of  the  real  curves.  The  diminution 
of  amplitude  is  almost  twice  as  great  in  the  direc- 
tion of  the  meridian  as  in  that  of  the  east-west 
direction  at  right  angles  to  it;  it  reaches  nearly 
half  in  the  direction  of  the  meridian.  The  little 
closed  loop  seen  on  the  two  curves  corresponding 
to  high  declinations  of  the  Moon  arises  from  the 
fact  that  there  are  two  daily  maxima;  these 
maxima  are  equal  if  the  Moon  is  in  the  plane  of 
the  equator;  inequal  if  it  is  to  the  north  or  south- 
of  the  equator,  the  more  so  as  it  is  farther  away 
from  the  equator. 

The  difference  between  the  calculated  and  ob- 
served curves,  assuming  that  the  latter  give  similar 
results  when  the  enquiry  is  pursued  over  a  longer 
space  of  time,  shows  that  the  Earth,  taken  in  its 
entirety,  possesses  a  certain  degree  of  elasticity 
which  is  of  the  same  order  of  magnitude  as  that 
of  steel.  In  other  words  the  Earth  behaves  almost 
as  if  it  were  made  of  solid  steel,  and  of  its  present 
dimensions.  It  is  especially  remarkable  that  the 
consideration  of  the  oceanic  tides,  the  astronomical 
movements  of  the  Earth,  and  the  displacement  of 
the  terrestrial  poles  all  lead  us  to  assign  an  elasti- 
city of  the  same  order  of  magnitude  to  the  Earth 
taken  as  a  whole;  it  is  an  admirable  confirmation 


Movements  of  tHe  EartH's  Crust     189 

of  the  original  idea  of  Lord  Kelvin.  There  is  only 
one  point  that  remains  obscure  in  the  interpreta- 
tion of  Dr.  Hecker's  results,  viz.,  the  reduction  of 
the  amplitude  of  the  deviations  in  the  direction 
of  the  meridian,  a  reduction  of  the  extent  of  the 
phenomenon  to  almost  half  its  value,  while  there 
is  scarcely  any  such  reduction  in  the  east-west 
direction.  In  the  masterly  analysis  of  this  ques- 
tion that  he  has  made,  Lallemand  has  sought  the 
cause  of  this  anomaly.  It  may  be  due  to  the 
instrument  itself,  or  to  the  relative  proximity  of 
the  sea,  or  to  a  peculiarity  of  structure  of  the 
Earth's  crust  in  the  Potsdam  region,  or  again  to 
the  tetrahedral  form  of  our  globe,  of  which  the 
Eurasian  arete,  oriented  in  the  east- west  direc- 
tion, passes  not  far  from  Potsdam.  Lallemand 
favours  this  last  suggestion.  In  spite  of  this  in- 
completeness of  our  knowledge  much  has  been 
achieved;  we  know  that  the  globe  taken  as  a 
whole  has  an  elasticity  of  the  same  order  of  magni- 
tude as  that  of  steel,  and  we  shall  see  later  on  that 
the  study  of  the  seismic  phenomena  brings  further 
confirmation  of  this  fact. 

As  a  result  of  our  study  of  the  combined  action 
of  gravity  and  the  luni-solar  attraction  on  a  plumb- 
line  we  can  deduce  another  consequence;  the 
thread  which  supports  the  mass  cannot  be  recti- 


190  The  Earth 

linear,  but  has  the  form  of  a  curve,  the  equation 
of  which  has  been  given  by  Puiseux.  This  curva- 
ture has  not  been  detected  by  any  of  our  methods 
of  measurement  so  far,  but  we  know  that  it  exists. 
Since  a  stretched  horizontal  thread,  however  fine 
it  may  be  and  however  well  it  may  be  stretched, 
is  never  rectilinear,  because  of  gravity  which 
imposes  upon  it  the  form  of  a  catenary,  it  will  be 
seen  that  a  straight  line  is  not  realisable,  at  any 
rate  mechanically.  It  is  also  the  same  optically; 
light,  because  of  the  movements  of  the  Earth, 
and  because  of  refraction  and  diffraction,  is  not 
propagated  in  a  straight  line.  The  idea  of  a 
luminous  ray  has  given  place  to  that  of  a  wave. 
The  edge  of  a  crystal  is  not  a  right  line,  for  during 
the  time  that  a  second  molecule  has  taken  to  align 
itself  with  the  first,  the  Sun  and  the  Moon  have 
changed  position  relatively  to  the  Earth  and  have 
deviated  the  molecule  -  from  the  position  it  would 
otherwise  have  taken. 

Is  the  straight  line  then  entirely  a  creation  of 
Man's  brain?  If  so,  he  might  be  justly  proud  of  it. 

However  this  may  be,  everything  about  our 
Earth  is  in  continual  movement,  in  spite  of  the 
deceptive  appearance  of  stability  presented  to  us. 
The  crust  expands  and  contracts  under  the  daily 
action  of  the  solar  heat;  the  nucleus,  rendered 


Movements  of  tKe  EartH's  Crust     191 

dense  and  compact  by  the  pressures  acting  upon 
it,  is  subjected  to  veritable  tides  under  the  influ- 
ence of  the  luni-solar  attraction  and  we  may  be 
certain  that  the  fluid  layer,  interposed  between  the 
nucleus  and  the  crust  which  covers  it,  is  agitated 
by  perpetual  movements,  both  of  tidal  and  con- 
vective  origin.  Where  then  may  we  find  real 
stability?  Where  is  the  invariability  which  the 
rocks  seemed  to  symbolise  so  well?  In  the  ima- 
gination of  poets  perhaps,  but  not  in  the  reality 
of  Nature,  where  everything  moves  perpetually. 
The  different  forms  of  movement  we  have  hitherto 
dealt  with,  whether  affecting  the  entire  Earth  or 
only  its  crust,  are  of  astronomical  origin.  We 
now  come  to  movements  of  a  different  nature, 
viz.,  the  sudden  movements  which  sometimes 
disturb  a  large  extent  of  the  Earth's  crust,  known 
as  earthquakes,  and  also  the  slow  continued 
movements  which  produce  the  raising  and  lowering 
of  the  crust. 


CHAPTER  VII 


THE  SUDDEN  MOVEMENTS  OF  THE  EARTH'S  CRUST. 
SEISMIC  PHENOMENA 


WHEN  the  terrestrial  crust  was  formed  by 
the  solidification  of  the  superficial  layers 
of  the  nucleus  of  fused  matter,  the  spheroidal 
form  of  which  constituted  the  Earth  at  the  com- 
mencement of  its  history,  a  colossal  reserve  of 
energy  was  imprisoned  inside  it.  This  energy 
results  from  the  heat  of  the  central  nucleus,  which 
exists  at  inconceivably  high  temperatures. 

Now  the  crust  is  far  from  homogeneous;  it  was 
not  formed  all  at  once,  but  in  pieces  of  which 
the  earlier  ones  constituted  scoriae,  isolated  and 
floating  on  the  surface  of  the  spherical  liquid 
mass.  These  became  gradually  united  with  each 
other  and,  being  of  various  thicknesses,  thus  formed 
the  first  irregularities  of  the  Earth's  crust,  which, 
as  previously  stated,  has  been  likened  by  Lappar- 
ent  to  a  marqueterie.  Its  lack  of  continuity  and 
homogeneity  involves  an  important  consequence; 

192 


Seismic  Phenomena  193 

if  we  compare  it  to  a  boiler,  this  boiler  will  not  be 
equally  strong  everywhere  but  will  have  weak 
places  in  its  sides,  flaws  as  they  are  called  in 
metallurgy,  and  it  will  burst  at  these  places  if 
the  interior  pressure  increases  beyond  a  certain 
limit. 

The  internal  energy  may  manifest  itself  by  the 
upward  expansion  of  the  material  forming  the 
superficial  layers  of  the  central  nucleus,  through 
a  fissure  in  the  crust.  Such  a  manifestation 
constitutes  a  volcanic  eruption. 

As  regards  the  cause  which  makes  this  material 
rise  up  through  the  fissures  and  fractures  of  the 
crust,  it  is  probable  that  under  the  influence  of  the 
progressive  cooling  of  the  nucleus,  a  cooling  which 
although  very  slow  continues  incessantly,  gases 
are  given  off  from  the  still  liquid  upper  layers  of 
the  internal  part,  and  accumulate  under  the  crust, 
upon  which  they  consequently  exert  a  pressure. 
Possibly,  also,  the  water  of  the  seas  infiltrates 
through  the  crust,  which  is  of  less  thickness  under 
the  seas  than  under  the  continents,  as  we  have 
already  seen;  such  infiltration  would  lead  to  the 
contact  of  the  water  with  the  igneous  masses  and 
the  consequent  dissociation  of  the  water  into  its 
constituent  gases.  This  would  be  another  cause 
of  an  increase  of  internal  pressure,  tending  to 


194  The  Earth 

break  open  the  crust  and  let  the  gases  and  the 
igneous  material  shoot  out,  or  in  any  case  to  disturb 
violently  the  sides  of  the  boiler,  so  to  speak,  which 
the  terrestrial  crust  forms.  The  thermal  energy 
accumulated  at  the  centre  can  thus  manifest  itself 
exteriorly  in  two  distinct  ways :  either  by  an  expan- 
sion of  the  inner  liquid  and  gaseous  material 
and  the  forcing  of  this  through  the  crust  which 
the  pressure  has  made  to  yield  at  some  point ;  or 
by  sudden  movements  and  agitations  imparted  to 
the  crust,  by  the  internal  pressure,  which  moves 
or  bends  the  crust  without  breaking  it,  this  some- 
times resulting  only  in  a  vibratory  phenomenon 
transmitted  as  true  waves.  The  first  is  a  volcanic 
eruption;  the  second  a  seismic  phenomenon. 
Volcanoes  and  earthquakes  are  consequently  two 
manifestations  of  the  same  cause,  but  they  are  in 
no  wise  directly  connected  together.  As  an  ex- 
ample, in  the  case  of  Japan,  which  is  the  classi- 
cal earthquake  country,  so  to  speak,  the  internal 
activity  which  is  constantly  manifested  by  very 
frequent  earthquakes  does  not  awake  the  old  vol- 
cano of  Fusiyama  from  its  long  quiescence. 

The  general  characteristic  of  a  volcano  is  that 
it  occupies  the  summit  of  a  mountain  and  gives 
off  permanently,  or  at  intervals,  a  greater  or  less 
abundance  of  vapours;  from  time  to  time  out  of 


Seismic  PHenomena  195 

an  opening  whose  mouth  is  at  the  summit,  and 
which  is  called  the  crater,  it  ejects  a  rain  of  stones 
and  cinders  accompanied  by  thick  clouds  of 
vapours  and  sometimes  by  burning  gases.  Such 
were  the  thick  burning  clouds  observed  at  Marti- 
nique during  the  courageous  and  profitable  study 
which  Professor  Lacroix  made  of  the  volcano, 
Mt.  Pelee.  These  clouds  are  often  the  seat  of 
violent  electrical  manifestations  and  are,  conse- 
quently, furrowed  with  lightning  flashes.  While 
these  emissions  of  gases  and  vapours  take  place 
into  the  atmosphere,  a  river  of  fused  rock  and 
similar  materials,  called  lavas,  emerges  from  the 
crater,  streams  down  the  sides  of  the  mountain, 
and  covers  the  surrounding  country,  sometimes 
to  a  considerable  distance,  retaining  for  a  long 
time  a  very  high  temperature. 

The  volcanic  mountains  have  been  formed  by 
such  lavas  and  eruptive  rocks.  The  first  eruption 
takes  place  through  a  fracture  in  the  crust;  the 
rocks  and  lava  accumulate  around  the  orifice  as 
an  ever-increasing  cone,  since  each  new  eruption 
expels  new  material  onto  the  flanks  of  the  small 
mountain  so  formed,  increasing  at  the  same  time 
its  extent  and  height.  Little  by  little  the  cone 
becomes  a  mountain,  which  remains  pierced  with 
a  passage  traversing  the  crust,  called  a  funnel, 


196  The  Earth 

preserving  communication  between  the  exterior 
of  the  crust  and  the  fluid  incandescent  magma 
which  forms  the  upper  part  of  the  central  nucleus. 
It  is  the  extremity  of  this  funnel  which  is  called 
the  crater. 

When  a  volcanic  mountain  attains  a  certain 
height,  for  example  over  4000  metres  [13,000  ft.], 
as  is  the  case  with  Mauna  Loa  in  the  Sandwich 
Isles,  it  needs  a  great  pressure  to  support  and  eject 
the  high  column  of  lava  which  fills  the  funnel. 
For  such  an  eruption  to  take  place,  the  cause  of 
which  we  have  already  spoken  must  operate,  viz., 
the  setting  at  liberty  of  enormous  quantities  of 
gases  in  the  upper  part  of  the  interior  magma. 
These  gases  result  from  the  effervescence  of  the 
fused  masses  and  are  expelled  violently  through 
the  orifice  open  to  them,  being  forced  out  in  con- 
sequence of  their  extreme  pressure.  In  the  case 
of  Mauna  Loa,  the  mountain  stands  on  the 
floor  of  the  Pacific  and  rises,  as  stated,  to  a 
height  of  more  than  4000  metres  [13,000  ft.]  above 
sea-level;  the  gases  which  eject  lava  to  its  summit 
must  therefore  exert  a  pressure  of  several  thousand 
atmospheres.  These  lavas  constitute  a  veritable 
lake  of  fire  in  the  great  crater  which  exists  at  the 
summit  of  the  mountain ;  they  overflow  and  spread 
in  great  rivers  of  fire  down  the  sides  of  the  gigantic 


Seismic  PHenomena  197 

cone.  We  have  here  a  typical  case  of  a  continuous 
outpouring  of  lava.  Such  a  volcano  is  a  true 
safety  valve  in  this  region  of  the  Earth's  crust. 

But  other  kinds  exist  which  are  subject  to  fre- 
quent eruptions,  notably  the  European  volcanoes 
such  as  Vesuvius  and  Etna.  When  the  lava  arrives 
at  the  summit,  already  cooler  and  in  less  quantity, 
or  when  after  an  eruption,  there  is  a  slackening  in 
the  upward  propulsion  of  the  fused  matter,  that 
which  fills  the  upper  part  of  the  crater  solidifies, 
and  the  internal  energy  can  then  manifest  itself 
again  only  by  the  emission  of  more  or  less  abun- 
dant gases  and  vapours,  which  are  prevented  from 
escaping  by  the  mass  blocking  the  funnel  of  the 
volcano,  and  so  accumulate  under  the  crust. 
Since  the  pressure  gradually  increases,  sooner  or 
later  it  becomes  greater  than  the  obstructing  mass 
can  sustain.  The  eruption,  therefore,  has  the 
character  of  a  veritable  explosion  and  often  the 
entire  mountain  bursts  like  a  shell  does  under  its 
charge  of  melinite.  The  debris  of  the  explosion 
is  projected  high  up,  ashes  being  carried  to  a  height 
of  several  kilometres  [or  miles],  and  rocks  are  flung 
on  to  the  surrounding  country  for  a  distance  of 
hundreds  of  kilometres  [or  miles].  The  eruption 
in  this  case  always  attains  the  character  of  a 
catastrophe;  it  will  suffice  to  recall  the  eruption 


198  The  EartH 

of  Mt.  Pelee  in  June,  1902,  and  that  of  Krakatoa, 
in  the  Sunda  Isles  in  1883.  In  cases  where  the 
volcano  rises  directly  out  of  the  sea,  constituting 
a  small  island,  the  latter  may  entirely  disappear 
in  the  course  of  the  cataclysm;  often,  however,  it 
only  partially  disappears,  leaving  only  the  crater 
above  the  sea-level.  In  this  case  a  pierced  or 
broken  portion  frequently  admits  the  sea  to  the 
interior  of  the  crater,  thus  forming  an  almost 
closed  bay  as  may  be  seen  in  Saint  Paul  Island, 
in  the  Southern  Seas.  This  crater-isle  has  been 
carefully  studied  by  Professor  Velain.  The  Greek 
Archipelago  has  often  been  the  seat  of  similar 
cataclysms,  for  example  that  of  Santorin. 

The  mass  of  material  ejected  by  volcanic  erup- 
tions may  attain  considerable  proportions.  This 
is  obvious  from  a  consideration  of  the  volcano  in 
the  Sandwich  Isles,  before  mentioned,  when  we 
recollect  that  the  mountain  itself,  more  than  4000 
metres  [13,000  ft.]  high,  is  formed  by  material 
ejected  during  successive  eruptions,  and  accumu- 
lated as  a  cone  around  the  original  orifice.  The 
island  itself,  which  is  entirely  constituted  of  lava, 
forms  a  mass  of  more  than  three  hundred  thousand 
cubic  kilometres,  since  the  cone  is  continued 
downwards  under  the  water  to  the  submarine 
floor  of  the  Pacific.  Even  Vesuvius,  one  of  the 


Seismic  PHenomena  199 

smallest  of  volcanoes,  has  given  forth  streams 
containing  fifteen  and  twenty  millions  of  cubic 
metres  [or  yards]  of  lava.  The  statement  of  these 
quantities,  if  we  remember  the  considerable  num- 
ber of  craters  that  are  active  at  the  present  time 
and  also  the  very  large  number  of  extinct  volca- 
noes, shows  that  the  exterior  appearance  of  the 
Earth  is  incessantly  modified  by  the  addition  of 
new  material  which  alters  its  relief  and  strews  its 
surface  with  mineral  matter  brought  from  the 
interior  of  the  globe. 

For  a  volcano  to  come  into  being,  there  must 
initially  be  a  cleft,  fissure,  or  crack  in  the  terrestrial 
crust.  Now  there  are  regions  of  the  Earth  which 
are  especially  prone  to  such  fractures,  viz.,  the 
border  of  the  oceans,  chiefly  those  where  an  ele- 
vated coastal  region  dips  suddenly  to  the  sea. 
The  seas,  in  fact,  mark  the  lowered  portions  of 
the  kind  of  marqueterie  formed  by  the  terrestrial 
crust  while  the  continents  represent  the  raised 
portions. 

Maritime  shores  are  therefore  volcanic  regions, 
places  given  over  to  volcanic  activity ;  it  is  only  ne- 
cessary to  glance  at  a  map  of  the  world  (Fig.  22,  p. 
215)  for  direct  confirmation  of  this  fact.  The  Pacific 
is  bordered  everywhere,  even  on  the  shores  of  the 
Antarctic  continent  by  a  girdle  of  active  volcanoes 


200  The  Earth 

which  constitute  a  veritable  fiery  circle;  so  also 
a  long  chain  of  volcanoes  lies  along  the  shores  of 
the  Mediterranean,  and  extends  by  Asia  Minor 
and  the  Persian  Gulf  to  the  Sunda  Isles.  Another 
line  of  craters  lies  in  the  midst  of  the  Atlantic  from 
Jean- May  en  and  Etna  in  the  north  to  the  Antarc- 
tic volcanoes  in  the  south  passing  by  the  shores 
of  the  Azores,  Madeira,  and  the  Canary  Islands, 
where  stands  the  imposing  Peak  of  Teneriffe  whose 
activity  has  recently  begun  to  manifest  itself 
anew. 

The  number  of  active  volcanoes  actually  known 
is  to  be  reckoned  by  hundreds,  and  this  does  not 
include  the  numerous  submarine  volcanoes  which 
doubtless  exist  beneath  the  oceans,  especially 
under  the  Pacific  and  whose  existence  and  activity 
are  only  made  known  to  us  by  abnormal  waves  on 
the  surface  of  the  oceans  which  cover  them.  Be- 
sides volcanoes  proper,  other  phenomena  at  various 
points  on  the  Earth's  surface  clearly  point  to 
internal  activity;  geysers,  hot  springs,  and  emana- 
tions of  sulphur  vapour  which  burst  through 
fissures  in  the  crust  bear  witness  to  the  heat  energy 
accumulated  in  the  interior  regions  below.  Vol- 
canic eruptions  and  gaseous  emanations  do  not 
always  form  a  sufficient  vent  for  the  manifesta- 
tions of  this  energy,  which  relieves  itself  by  means 


Seismic  Phenomena  2OI 

of  other  phenomena  that  we  will  now  study,  viz., 
seismic  phenomena. 

The  causes  of  the  instability  of  the  Earth's 
exterior  envelope  are  numerous.  We  have  noted, 
in  the  course  of  preceding  chapters,  those  of  them 
that  are  periodic,  but  there  are  others,  the  expla- 
nation of  which  must  be  sought  in  the  situation  of 
the  crust  itself  in  relation  to  the  heated  nucleus 
which  it  covers. 

This  nucleus  cools  at  a  constant  rate,  and  in  so 
cooling  contracts.  In  the  course  of  time,  therefore, 
a  space  would  be  left  between  the  upper  layer  of 
the  nucleus  and  the  lowest  part  of  the  crust  which 
floats  on  its  surface.  The  part  of  the  crust  which 
is  immersed  in  the  liquid  mass  below  it  has  in  all 
cases  an  upward  thrust  exerted  upon  it  by  this 
mass.  Now  this  thrust  will  diminish  in  proportion 
as  the  internal  mass  contracts.  After  a  sufficient 
interval  of  time  the  crust  becomes  insufficiently 
supported  from  below  and  so  tends  to  sink  down, 
and  this  sinking  gives  rise  to  a  disturbance  which 
affects  a  larger  or  smaller  area  around  the  prin- 
cipal centre  of  action.  As  the  cause  operates 
continually,  this  result  will  occur  at  all  times  of 
the  Earth's  history,  though  perhaps  in  a  discon- 
tinuous way. 

This  is  not  all.     Volcanic  eruptions  throw  on 


202  The  Earth 

to  the  surface  of  the  Earth's  crust  a  quantity  of 
material  which  was  formerly  below  it;  such  mate- 
rial is  not  replaced  and  its  removal  creates  a  space 
below  the  crust  and  at  the  same  time  adds  to  the 
weight  of  the  latter.  Gravity  consequently  tends 
to  make  the  surface  fall  in  and  fill  up  the  space 
when  the  upward  thrust  from  below  is  insufficient, 
and  this  constitutes  another  reason  why  the  exter- 
nal envelope  of  the  Earth  sinks. 

It  is,  therefore,  to  be  expected  that  each  such 
sinking  will  be  manifested  by  a  shock,  sometimes 
feeble  and  sometimes  great,  according  to  the 
degree  of  the  fall  which  produces  it.  Furthermore, 
currents  circulate  in  the  upper  regions  of  the  cen- 
tral nucleus,  the  part  that  is  still  fluid,  and  so 
cause  waves  and  undulatory  movements  which 
come  into  contact  with  the  parts  of  the  crust  which 
project  below  its  interior  surface.  The  force  of 
these  agitations  results  in  the  shaking  of  these 
projecting  parts,  and  the  disturbance  is  transmitted 
by  them  to  the  rest  of  the  solid  crust.  There 
are  thus  numerous  reasons  why  the  crust  is  never 
in  repose. 

In  consequence  of  the  incessant  disturbances 
to  which  the  crust  is  subjected,  the  importance 
of  the  study  of  these  sudden  movements  that  have 
such  a  disturbing  effect  upon  it  will  be  obvious. 


Seismic  PHenomena  203 

Some  of  the  shocks  are  devastating,  others  are 
feeble,  sometimes  even  so  feeble  that  only  delicate 
instruments  called  seismographs,  which  are  always 
based  on  the  principle  of  inertia,  can  disclose  and 
register  them.  There  are,  therefore,  earthquakes 
and  earth  tremors,  of  varying  intensity. 

It  is  customary  to  divide  the  shocks  which  the 
Earth's  crust  undergoes  into  three  categories: 
vertical  shocks  which,  if  intense  enough,  may 
project  buildings  upward  into  the  air,  as  if  an 
explosion  had  taken  place ;  horizontal  shocks  which 
displace  objects  on  the  ground  laterally  and  which 
are  capable,  among  other  results,  of  displacing 
an  upper  course  of  masonry  with  respect  to  a 
lower  one;  and  finally  undulatory  shocks,  the 
most  numerous  and  the  most  terrible,  which 
spread  through  the  ground  surface  in  the  same  way 
as  the  swell  of  the  ocean  spreads  through  the  water 
surface.  When  such  a  seismic  wave  occurs  the 
surface  of  the  Earth's  crust  is  agitated  and  dis- 
turbed just  as  the  waves  of  the  sea  are.  But  these 
shocks  produce  a  permanent  alteration  in  the 
solid  surface,  whereas  the  waves  of  the  sea  give 
rise  to  only  a  passing  perturbation ;  numerous  deep 
crevices  appear,  buildings  are  destroyed,  trees  torn 
up,  and  whole  towns  may  be  annihilated.  Recent 
examples  are  Valparaiso,  San  Francisco,  'and,  still 


204  The  EartH 

more  lately,  Messina,  where  the  earthquake 
destroyed  more  than  200,000  human  lives  in  a 
few  seconds. 

The  centre  of  disturbance,  the  point  from  which 
the  waves  seem  to  radiate,  is  almost  always  below 
the  surface  of  the  ground,  sometimes  at  very  con- 
siderable depths,  even  up  to  20  kilometres  [12.5 
miles].  This  point  is  analogous  to  that  where  a 
stone  thrown  into  water  strikes  the  water;  circular 
waves  originate  there  and  travel  outwards.  The 
orientation  of  the  crevices  and  their  inclination 
to  the  vertical  enable  the  position  of  this  point 
to  be  fairly  accurately  obtained.  The  projection 
of  the  centre  of  disturbance  onto  the  ground 
surface,  that  is  to  say  the  place  where  the  centre 
would  be  marked  on  a  map,  is  commonly  called 
the  epicentre.  The  crevices  crop  out  in  the 
ground  around  the  epicentre,  forming  concentric 
curves,  roughly  circular  when  there  is  only  one 
centre  of  disturbance,  but  often  elongated,  in 
which  case  the  existence  of  several  such  centres 
seems  to  be  indicated.  The  movements  are  pro- 
pagated at  the  surface  of  the  ground  with  veloci- 
ties varying  between  150  and  800  metres  [500- 
2500  ft.]  per  second;  we  shall  see  later  on  that  the 
rate  of  propagation  for  the  total  mass  of  the  Earth 
is  much  more  rapid. 


Seismic  Phenomena  205 

These  great  disturbances  are  happily  not  very 
frequent;  it  is  the  earth  tremors,  detected  and 
registered  only  by  means  of  seismographs,  which 
by  their  frequency  prove  the  continual  quivering 
of  the  solid  crust  of  the  Earth.  It  appears  that 
more  violent  earthquakes  occur  when  the  baro- 
metric pressure  is  low,  which  is  easily  understand- 
able, since  the  terrestrial  crust  supports  a  less 
quantity  of  atmosphere  than  usual  and  so  the 
pressure  inside  it  is  not  so  much  opposed  as  usual. 
A  barometric  fall  of  I  centimetre  [.39  in.]  produces 
an  increase  of  internal  pressure  of  130  kilograms 
per  square  metre  [285  Ibs.  per  sq.  yd.],  i.  e.,  130 
millions  of  kilograms  per  square  kilometre.  Earth- 
quakes are  also  more  frequent  in  winter  than  in 
summer  and  are  especially  numerous  at  the  time 
of  the  equinoxes;  the  eruption  of  Mt.  Pelee,  in 
Martinique,  in  1902,  accompanied  by  a  consider- 
able local  earthquake  and  a  tidal  wave,  occurred 
at  a  time  when  the  Sun  and  the  Moon  were  in  a 
straight  line  with  the  Earth  and  so  produced  a 
combined  attractive  effect  on  the  latter.  Possibly 
internal  tides  arise  forming  a  wave  at  the  upper 
liquid  surface  of  the  central  magma;  it  is  then 
readily  understandable  that,  at  the  period  of  the 
equinoxes,  when  the  luni-solar  attraction  is  great- 
est, the  internal  tide,  and  consequently  its  wave- 


206  TKe  EartK 

force,  would  be  strongest.  In  this  case,  as  M. 
Kovesligethy  believes,  external  factors  would  be 
the  determining  causes  of  the  liberation  of  the 
internal  energy  which  takes  place  in  virtue  of 
the  weaknesses  of  the  crust. 

It  is  possibly  in  this  direction  that  we  must 
seek  the  solution  of  the  important  problem  of  the 
foretelling  of  earthquakes;  such  a  result  can  only 
be  attained  by  studying  the  laws  which  govern  the 
movements  of  the  superior  fluid  layer  of  the  inte- 
rior nucleus  of  the  Earth,  with  the  aid  of  modern 
physical  methods,  with  their  increasing  precision. 
A  remarkable  coincidence  between  the  years  of 
maximum  earthquakes,  of  maximum  polar  aurorae, 
and  of  maximum  magnetic  storms  has  already 
been  demonstrated.  The  periodicity  of  the  three 
phenomena  is  the  same,  viz.,  eleven  years,  which 
is  also  the  periodicity  of  the  maximum  activity 
of  the  solar  spots.  Our  Sun,  by  the  attraction 
of  its  mass,  is  the  cause  of  the  complex  movements 
the  Earth  performs;  by  warming  the  Earth's 
crust  it  produces  a  daily  deformation  of  the  latter; 
it  causes  tides,  not  only  at  the  surface  of  the  seas, 
but  also  at  the  surface  of  the  ocean  of  heated  lava 
which  exists  beneath  our  feet.  The  question  natu- 
rally follows  whether  the  periodical  variation  in 
th?  number  of  the  solar  spots  produces  a  variation 


Seismic  PHenomena  207 

of  the  kinds  of  radiation  emitted  by  the  Sun  and 
what  effect  this  would  have  upon  the  Earth.  The 
Sun  creates  a  field  of  force  about  it  in  space,  and 
the  intensity  of  this  field  is  affected  by  the  slight- 
est variations  in  the  solar  activity.  It  may  be, 
therefore,  that  we  must  make  a  fuller  study  of  the 
Sun  in  order  to  determine  the  law  of  the  vicissitudes 
of  the  Earth's  thin  and  incessantly  quivering  crust. 
When  we  have  described  the  magnetic  and  electri- 
cal phenomena  of  which  the  Earth  is  the  seat,  we 
shall  still  better  understand  the  unquestionable 
influence  that  the  Sun  exerts  on  the  terrestrial 
globe. 

Seismic  phenomena  should  not  be  treated  as 
isolated  occurrences,  for  the  same  reason  that 
applies  in  the  case  of  volcanic  eruptions,  viz., 
that  they  are  different  manifestations  of  the  in- 
ternal energy,  having  no  "laws"  or  necessary 
interconnection  in  time  or  space,  but  they  never- 
theless arise  from  one  sole  cause,  so  some  law 
should  govern  them  when  taken  together. 

The  universal  prevalence  of  seismic  phenomena 
is  established,  just  as  in  the  case  of  volcanic  erup- 
tions. As  regards  the  last,  we  know  that  nearly 
four  hundred  active  craters  exist  on  the  Earth's 
surface,  and  that  more  than  double  this  number 
of  extinct  or  sleeping  ones  can  be  distinguished, 


208  THe  Earth 

and  this  takes  no  account  of  the  unknown  num- 
ber, which  is  perhaps  very  large,  that  the  oceans 
cover  with  their  vast  area  of  water.  In  recent 
years  there  have  been  signs  of  awakening  of  many 
of  these  centres  of  eruption,  for  example,  in  1909, 
as  has  previously  been  mentioned,  the  old  volcano 
of  Teneriffe,  which  had  seemed  definitely  extinct 
has  given  proof  of  a  renewal  of  activity. *  A  week 
never  passes  without  the  telegraph  bringing  news 
from  some  part  of  the  world  of  earth  movements, 
sometimes  devastating,  sometimes  less  important, 
but  always  clearly  perceptible;  especially  in  Tur- 
kestan, India,  the  Caucasus,  the  Philippine  Isles, 
Japan,  Sicily,  and  Provence,  the  Earth  quivers  and 
suffers  incessant  disturbance.  Islands  even  dis- 
appear suddenly.  We  have  thus  isolated  occur- 
rences which  are  various  forms  of  phenomena  all 
resulting  from  one  general  cause.  Lallemand  has 
investigated  whether  it  would  not  be  possible  to 
account  for  the  seismic  manifestations  of  the  inter- 
nal activity  on  the  basis  of  the  tetrahedral  theory 
of  the  formation  of  the  terrestrial  crust,  which 
theory  was  proposed  by  Lowthian  Green  in  1875. 
We  have  already  said  a  few  words  about  it  near 


xln  June,  1914,  Lassen's  Peak  in  California  became  mildly 
active,  though  for  many  years  it  had  been  considered  entirely 
extinct.— Ed. 


Seismic  Phenomena  209 

the  beginning  of  this  work,  and  we  must  now  return 
to  it  in  fuller  detail.  The  English  scientist  had 
shown  that  when  tubes  of  india-rubber  were  com- 
pressed from  the  outside,  these  tubes  instead  of 
being  flattened  took  a  form,  the  section  of  which 
was  a  triangle  with  concave  sides.  If  the  air  con- 
tained in  a  glass  globe,  which  is  softened  by  heat, 
be  exhausted,  the  globe,  originally  spherical, 
takes  a  form  in  which  four  hollow  faces  are  clearly 
seen,  these  being  the  regions  of  flattening  under 
the  influence  of  the  relative  external  increase  of 
pressure.  This  form  of  triangular  pyramid,  or 
rather  the  tendency  to  take  this  form,  is,  more- 
over, a  consequence  of  the  principle  of  least  action ; 
starting  with  a  fixed  surface  area,  the  terrestrial 
crust  should,  nevertheless,  diminish  as  regards  its 
enclosed  volume,  since  the  force  of  gravity  makes 
it  remain  in  contact  with  the  internal  nucleus  and 
since  this  is  continually  contracting  as  it  cools. 
In  order  to  enclose  a  minimum  volume,  and  at  the 
same  time  obey  the  double  condition  of  maintain- 
ing a  fixed  surface  area  and  a  symmetrical  form, 
the  crust  must  tend  appreciably  towards  the  figure 
of  a  regular  tetrahedron,  i.  e.,  a  triangular  pyramid 
with  equilateral  faces,  which  is  a  regular  solid 
occupying  the  minimum  volume  for  a  given  area 
of  surface. 

M 


210  THe  EartK 

It  seems,  however,  at  first  sight,  that  the  pyrami- 
dal form,  with  its  edges,  apices,  and  faces,  is  far 
removed  from  a  spheroidal  one,  but  we  shall  see 
that  such  dissimilarity  is  only  apparent  and  that, 
on  the  contrary,  the  resemblance  becomes  marked 
when  we  study  the  matter  more  closely. 

The  exterior  appearance  of  the  Earth,  i.  e.,  the 
aspect  it  would  present  to  an  observer  placed  far 
away  from  it  in  space,  is  the  result  of  the  combina- 
tion of  the  solid  crust  and  its  aqueous  envelope, 
or  in  other  words  the  lithosphere  and  the  hydro- 
sphere, the  barysphere  or  central  nucleus  being 
in  the  interior  of  the  first  two. 

If,  since  the  time  of  its  definite  solidification, 
the  crust  tended  to  take  the  tetrahedral  form,  its 
foldings,  and  consequently  the  general  orientation 
of  the  features  of  its  relief,  would  have  been  made 
under  the  influence  of  that  tendency  (Fig.  4,  p.  32 ). 
Hence  the  regions  near  the  summits  of  the  pyramid 
would  be  the  only  ones  emerging  above  the  hydro- 
sphere. Moreover  it  is  natural  to  suppose  that 
the  terrestrial  axis  coincides  with  one  of  the  four 
axes  of  symmetry  of  the  tetrahedron ;  there  ought, 
thus,  to  exist  in  one  of  the  hemispheres  three 
continental  elevations,  represented  by  three  sum- 
mits, the  corresponding  pole  being  occupied  by  an 
ocean,  the  bottom  of  which  is  represented  by  one 


Seismic  Phenomena  211 

of  the  flattened  faces  of  the  pyramidal  figure.  On 
the  other  hand,  the  opposite  pole  would  be  at  the 
fourth  summit  of  the  pyramid  and  consequently 
a  continental  mass  would  emerge  there  above  the 
spheroidal  surface  of  the  oceans. 

Voyages  made  in  the  polar  regions,  both  arctic 
and  antarctic,  during  recent  years  fully  confirm 
these  aspects  of  the  theory.  Nansen,  in  the  course 
of  his  circumnavigation  around  the  North  Pole 
has  shown  that  that  region  was  occupied  by  a  sea 
whose  depth  reaches  nearly  4000  metres  [2.5 
miles];1  on  the  other  hand  Ross,  de  Gerlache, 
Charcot,  Scott,  Shackleton,  and  Amundsen  have 
verified  the  existence  around  the  South  Pole  of  an 
immense  continent  whose  centre  is  occupied  by 
a  highly  elevated  plateau  and  above  which  rise 
peaks  whose  height  surpasses  4000  metres.  The 
diametrical  opposition  of  the  continents  and  seas 
is  consequently  demonstrated  with  remarkable 
clearness,  as  regards  the  polar  regions. 

It  is  equally  verified  by  terrestrial  geography  as 
a  whole;  the  three  continents  Europe,  Asia,  and 
America,  widened  at  the  north  and  narrowed  to- 

1  Peary,  in  1909,  when  on  his  successful  trip  to  the  North  Pole, 
secured  much  new  information  about  the  Arctic  Sea.  In  support 
of  the  present  theory,  he  failed  to  reach  bottom  on  his  sounding 
made  farthest  north — within  about  five  miles  of  the  pole  itself — 
although  using  a  line  of  1500  fathoms  [9000  ft.]  in  length. — Ed. 


212  The  Earth 

wards  the  south,  are  separated  by  three  oceans, 
narrowed  in  the  northern  parts  and  broadening  in 
the  southern  hemisphere.  It  may  be  said  that 
Europe  and  Asia  are  connected  together  in  their 
northern  parts,  but  this  is  rather  a  superficial 
objection,  since  beyond  the  Caspian  Sea  and  the 
Sea  of  Aral  obvious  signs  of  an  actual  depression 
between  these  two  continents  exist.  Also  precise 
measurements  have  shown  that  the  western  half 
of  Siberia  has  only  a  very  slight  elevation  above 
sea-level ;  a  very  slight  lowering  of  the  level  would 
transform  that  part  of  the  continent  into  a  sea. 
Possibly,  at  a  not  very  far  distant  period  this 
depression,  lying  along  the  foot  of  the  Ural  Moun- 
tains, was  covered  by  an  actual  sea.  The  pointed 
terminations  of  the  continents  towards  the  south, 
Cape  Horn,  the  Cape  of  Good  Hope,  the  point  of 
Tasmania  prolonging  Australia,  which  is  itself  a 
continuation  of  the  Asiatic  continent,  indicate 
that  the  base  of  the  terrestrial  tetrahedron  is 
towards  the  north.  The  northern  widened  parts 
of  America  and  Asia  are  very  nearly  connected 
together  by  the  elongations  between  which  passes 
the  Strait  of  Behring. 

But  we  can  carry  still  further  the  conclusions 
that  may  be  deduced  from  this  theory  of  the  ter- 
restrial tetrahedron.  So  far,  we  have  only  con- 


Seismic  PHenomena  213 

sidered  the  tendency  to  the  tetrahedral  form  in 
the  case  of  an  immovable  Earth.  We  know, 
however,  that  the  Earth  is  not  immovable,  but, 
on  the  contrary,  performs  a  number  of  combined 
movements  of  which  one  of  the  most  important 
is  its  movement  of  rotation. 

What  would  be  the  effect  of  the  Earth's  rotation 
on  the  tetrahedral  figure  at  the  time  when  this 
was  being  formed?  We  shall  see  that  it  would 
deform  the  lines  and  produce  on  the  solid  crust  a 
geographical  modification  of  which  indisputable 
evidence  is  found  and  which  would  be  difficult  to 
explain  in  any  other  way.  A  familiar  comparison 
will  lead  us  to  understand  the  nature  and  origin 
of  this  deformation. 

Let  us  take  an  old  umbrella,  the  covering  mate- 
rial of  which  has  been  removed,  and  at  the  end  of 
each  rib  attach  a  little  leaden  ball.  Then  let  the 
umbrella  be  opened  and  an  effort  be  made  to  make 
it  turn  between  the  fingers,  holding  it  vertically 
with  one  hand  and  rotating  the  curved  part  of  the 
handle  with  the  other;  this  curved  part  furnishes 
a  lever  arm  to  the  motive  force  given  by  the  hand. 
A  resistance  will  be  felt  which  tends  to  retard  the 
rotation  of  the  umbrella  and  such  resistance  is 
due  to  the  moment  of  inertia  of  the  apparatus, 
this  tending  to  resist  the  turning  force  that  we 


214 


The  Earth 


apply.  If  we  apply  a  too  violent  force  to  the 
handle  in  this  way,  we  shall  cause  a  torsion  of  the 
ribs  and  their  supporting  pieces  and  these  will 
twist  if  their  attachment  be  sufficiently  strong  to 
stand  it. 

A  similar  thing  occurred  at  the  time  of  the  solid- 
ification of  the  crust.     The  emergent  continental 


FIG.  21. — The  Deviation  of  the  Southern  Continents  toward 
the  East. 

apices,  though  acted  upon  by  the  force  of  rotation, 
remained  in  place  by  reason  of  their  inertia.  But 
the  influence  of  the  rotation  continued  to  make 
itself  felt  and  the  edges  connecting  the  northern 
continental  apices  with  the  South  Polar  apex  (Fig. 
4,  p.  32)  were  twisted  in  the  middle  so  that  while 
the  northern  parts  of  the  continents  remained 
retarded,  towards  the  west,  their  southern  parts, 
narrowing  down  to  points,  became  deflected  to- 


Seismic  PHenomena 


215 


wards  the  east  because  of  a  common  deviation. 
A  glance  at  a  map  of  the  world  (Fig.  21)  clearly 
shows  the  fact  of  this  deviation. 

Now,  in  twisting,  the  edges  of  the  tetrahedron 
became  weakened  in  their  middle  points  and  the 
crust  gave  way  there ;  this  line  of  rupture  actually 
exists,  and  has  received  from  geographers  the 


FIG.  22. — The  Distribution  of  Volcanic  Regions — The  Inter- 
continental Depression. 

name  of  the  intercontinental  depression.  This 
depression  is  a  kind  of  furrow,  a  marine  girdle 
which  completely  surrounds  the  terrestrial  globe 
nearly  at  its  middle,  that  is  to  say  in  the  neigh- 
bourhood of  the  equator,  of  which  it  is  north 
in  some  parts  and  south  in  others.  Europe,  in 
fact,  is  separated  from  Africa  by  the  Mediter- 
ranean Sea;  Asia  is  separated  from  Australia  by 
a  series  of  seas  almost  blocked  with  chains  of  islands 


2i6  The  Earth 

which  are  mountains  whose  summits  only  appear 
above  the  waters.  Finally  North  America  is  con- 
nected to  South  America  only  by  the  frail  junc- 
tion known  as  the  Isthmus  of  Panama  (Fig.  22). 

The  remarkable  conception  of  a  terrestrial 
tetrahedron  has  yet  another  consequence;  it 
allows  of  a  very  simple  explanation  of  the  dis- 
tribution of  volcanoes  and  of  centres  of  seismic 
disturbances  on  the  Earth's  surface. 

When  the  interior  crust,  under  the  influence 
of  the  contraction  of  the  central  nucleus,  became 
folded  and  wrinkled  in  order  to  remain  in  contact 
with  the  nucleus,  which  gravity  forces  it  to  do, 
the  foldings  showed  the  tendency  to  conform  to 
the  tetrahedral  character.  These  foldings  were 
formed  in  a  still  plastic  crust,  but  later,  when  it 
became  rigid,  the  action  of  the  same  forces  tended 
to  produce,  not  foldings,  but  fractures.  Hence 
the  continuous  shocks  which  disturb  the  crust  are 
due  to  its  deformation. 

But  the  regions  where  the  foldings  were  pro- 
duced are  the  regions  of  least  resistance ;  if  a  boiler 
plate  be  bent  and  then  made  use  of,  the  pressure 
of  the  steam  will  cause  a  fracture  exactly  in  the 
place  where  it  was  bent.  Now  the  edges  of  the 
tetrahedron  and  the  neighbouring  regions  are 
the  foldings  of  the  crust,  and  so  its  resistance  ought 


Seismic  Phenomena  217 

to  be  more  feeble  there.  The  same  applies  to  the 
whole  length  of  the  intercontinental  depression 
where  the  crust  has  already  suffered  a  twisting 
tending  to  enfeeble  its  resistance  to  rupture.  The 
great  continental  ridges,  such  as  the  American 
Cordilleras  and  the  chains  of  islands  bordering 
the  Pacific,  will  thus  be  the  special  regions  of 
earthquakes  and  of  volcanoes,  the  latter  formed 
about  the  fissures  of  the  folded  and  weakened 
crust.  A  fortiori,  the  greatest  number  of  volca- 
noes ought  to  be  situated  at  the  points  of  inter- 
section of  the  continental  ridges  with  the  great 
intercontinental  depression.  That  this  is  so  may 
be  seen  at  once  from  a  glance  at  a  map  giving  the 
distribution  of  volcanoes  over  the  Earth's  surface 
(Fig.  22) .  The  Pacific  in  particular  is  surrounded 
by  a  veritable  fiery  circle.  On  the  other  hand, 
craters  are  not  met  with  on  the  gentle  slopes  whose 
uniform  inclination  shows  that  there  have  been 
no  foldings  and  sudden  deformations. 

The  mining  engineer,  M.  Lallemand,  to  whom 
we  owe  the  extremely  precise  methods  of  modern 
determinations  of  altitude,  has  shown  that  the 
tetrahedral  theory  also  allows  of  a  very  natural 
explanation  of  the  anomalies  that  have  been  proved 
to  exist  in  the  values  of  gravity.  As  we  know, 
gravity  is  weakest  in  the  middle  of  a  continent 


2i8  The  EartK 

and  strongest  on  the  oceanic  islands,  whereas  if 
we  were  guided  only  by  the  consideration  of  the 
density  of  the  immediately  neighbouring  regions, 
the  reverse  should  be  the  case. 

Actually,  the  exterior  of  the  terrestrial  globe 
comprises  two  different  things,  the  lithosphere 
which  is  the  foundation,  supporting  the  hydro- 
sphere which  covers  the  greater  part  of  it.  The 
latter,  because  of  its  fluidity,  obeys  the  combined 
actions  of  gravitation  and  centrifugal  force.  If 
its  surface  be  prolonged  in  imagination  beneath 
the  earth,  as  a  result  of  the  operations  of  deter- 
mination of  altitude,  we  arrive  at  a  surface  called 
the  geoid,  as  previously  stated,  which  is  the  fun- 
damental surface  of  elevation  in  the  consideration 
of  gravity.  But  in  the  neighbourhood  of  the 
summits  of  the  tetrahedron,  that  is  to  say,  in  the 
central  regions  of  the  great  continental  masses, 
this  surface  must  project  above  the  normal  ellip- 
soid of  the  geodesists,  for  the  tendency  to  the  tet- 
rahedral  form  manifested  by  the  lithosphere  since 
its  original  solidification  ought  also  to  be  found  in 
a  smaller  degree,  in  the  fundamental  surface  of 
elevation.  Consequently  there  should  be  corre- 
sponding irregularities  in  the  values  of  gravity 
reduced  to  sea-level,  that  is  to  say,  after  allowance 
has  been  made  for  the  attraction  of  the  subjacent 


Seismic  PHenomena  219 

crust.  In  the  neighbourhood  of  the  edges  of  the 
tetrahedron,  rising  above  the  theoretical  ellipsoid, 
the  attractive  force  ought  therefore  to  be  weaker 
and  the  centrifugal  force  stronger  than  in  the 
middle  of  the  oceans,  where  the  stronger  attrac- 
tion and  the  weaker  centrifugal  force  produce, 
on  the  contrary,  an  excess  of  gravity.  This  excess 
in  the  oceanic  islands  and  the  deficit  in  the  interior 
of  the  continents  are  shown  by  actual  experimental 
results. 

This  new  explanation  due  to  M.  Lallemand  is 
in  complete  accord  with  the  theory  of  Professor 
Lippmann.  It  is  also  another  confirmation  of 
Lowthian  Green's  conception  of  the  terrestrial 
tetrahedron.  One  last  source  of  confirmation 
has  not  yet  been  realised,  but  will  certainly  be  so 
sooner  or  later,  thanks  to  the  efforts  of  the  Inter- 
national Geodetic  Association,  viz.,  the  measure- 
ment of  three  long  meridian  arcs  in  the  southern 
hemisphere,  as  near  as  possible  to  the  South  Pole 
and  in  particular  in  the  Argentine  and  the  Antarc- 
tic continent.  If  the  Earth  has  really  tended  to  a 
tetrahedral  form  in  becoming  externally  solidified, 
and  considering,  besides,  the  large  proportion  of 
water  and  the  small  extent  of  land  in  this  hemi- 
sphere, it  may  legitimately  be  supposed  that  the 
flattening  of  the  Earth  will  be  a  little  less  in  the 


220  The  Earth 

southern  hemisphere  than  in  the  northern  one. 
The  greater  number  of  the  long  meridian  arcs  on 
which  geodesists  have  centred  their  efforts  during 
the  last  century  and  a  half  are  situated  in  the 
northern  hemisphere.  The  Peru  arc,  measured 
in  the  eighteenth  century  by  Bouguer  and  La 
Condamine,  and  remeasured  recently  with  great 
trouble  and  remarkable  precision  by  the  equato- 
rial mission  under  the  direction  of  Colonel  Bour- 
geois, and  the  arc  of  the  Cape  of  Good  Hope,  the 
work  of  English  astronomers,  are  the  only  ones 
furnishing  us  with  data  for  the  southern  hemi- 
sphere. It  is  to  be  hoped  that  a  continuous  arc, 
from  Cape  Horn  to  the  equator,  will  soon  be 
measured  in  South  America.  Then  we  shall  per- 
haps have  the  ultimate  data  that  we  lack,  in 
default  of  a  method  and  instrument  permitting 
navigators  to  measure  the  intensity  of  gravity 
with  precision  on  board  ship  in  the  open  sea,  in 
the  regions  of  the  geoid  where,  up  to  the  present, 
all  accurate  results  have  been  found  impossible. 

The  scientific  observation  of  earthquakes,  which 
registering  seismographs  enable  nowadays  to  be 
made  continuously,  gives  us  new  information  as 
to  the  rigidity  of  the  terrestrial  globe,  which  as 
previously  shown,  we  have  deduced  from  the 
tides  of  the  crust. 


Seismic  Phenomena  221 

When  a  strong  earth  tremor  is  produced  at  any 
point  whatsoever  of  the  Earth,  the  most  distant 
seismographical  observatories,  those  which  for 
example  are  situated  6000  or  8000  kilometres 
[3600  to  4800  miles]  from  the  initial  centre  of  dis- 
turbance, are  affected  by  it  after  several  minutes, 
when  the  seismographs  become  agitated.  If  the 
time  of  the  first  registering  of  the  phenomenon, 
propagated  through  the  entire  mass  of  the  Earth 
and  not  simply  over  the  surface  of  the  crust,  be 
compared  with  the  actual  time  of  its  occurrence 
at  the  place  of  origin,  it  may  be  proved  that  the 
movement  is  propagated  with  a  velocity  of  about 
10  kilometres  [6.2  miles]  per  second.  This  is  a 
speed  three  hundred  times  greater  than  that  of  the 
most  rapid  of  our  express  trains. 

After  a  further  few  minutes,  the  apparatus  will 
be  again  disturbed,  more  strongly  and  for  a  greater 
length  of  time  than  previously.  If,  as  in  the 
preceding  case,  we  compare  the  times  of  origina- 
tion and  registering  of  the  original  shock  we  find 
that  these  other  seismic  waves  are  propagated  with 
a  velocity  of  5  kilometres  [3  miles]  per  second, 
viz.,  about  half  of  the  preceding.  If  we  compare 
these  results  with  those  which  the  mathematical 
theory  of  elasticity  gives,  we  find  an  exact  agree- 
ment. In  fact  this  theory,  which  is  based  on 


222  The  EartK 

experimental  evidence,  teaches  us  that  if  an  in- 
stantaneous disturbance  be  produced  at  one  point 
in  a  perfectly  elastic  solid,  two  series  of  waves  arise 
in  the  solid,  the  first  of  which  propagate  themselves 
with  a  velocity  double  that  of  the  second.  This  is 
precisely  what  the  study  of  seismographical  ob- 
servations shows  us,  and  so  we  have  a  remark- 
able accordance  between  theory  and  observation. 

By  making  use  of  the  seismographical  results 
in  the  elasticity  calculations  it  is  found  that  the 
elasticity  of  the  Earth,  considered  in  its  entirety, 
is  of  the  same  order  of  magnitude  as  that  of  steel, 
actually  a  little  greater.  This  agrees  wonderfully 
with  the  results  deduced  from  the  study  of  the 
terrestrial  tides  and  of  nutation. 

We  can  now  understand,  from  this  knowledge 
of  the  Earth's  elasticity,  why  observations  of  the 
propagation  of  seismic  disturbances  to  the  imme- 
diately neighbouring  regions  has  never  shown  a 
velocity  of  more  than  800  metres  [i  a  mile]  per 
second.  Waves  are  transmitted  to  the  neighbour- 
ing points  by  the  crust  itself,  while  transmission 
to  distant  places  takes  place  through  the  elastic 
medium  constituted  by  the  Earth  as  a  whole. 
Thus  the  density  of  the  central  nucleus  of  our 
globe  is  confirmed;  although  at  inconceivable 
temperatures  this  acquires,  by  reason  of  the 


Seismic  PHenomena  223 

pressure  to  which  it  is  subjected,  a  physical  state 
practically  equivalent  to  the  solid  state,  and  con- 
sequently possesses  a  rigidity  of  the  same  order  as 
that  of  the  best  kinds  of  steel. 

Earthquakes  result  not  only  in  sudden  shocks 
but  also  in  permanent  deformations  of  the  ter- 
restrial crust.  We  must  seek  for  the  origin  of 
these,  not  in  explosive  eruptions,  but  in  settle- 
ments, i.  e.,  in  movements  which  affect  the  jux- 
taposed portions  of  the  marqueterie  which  the 
terrestrial  crust  resembles,  when  such  portions 
exhibit  a  certain  amount  of  play  with  regard  to 
each  other.  This  conclusion  is  verified  by  the 
permanent  cracks  which  accompany  great  earth- 
quakes and  which  sometimes  attain  up  to  50  or 
even  100  kilometres  [30  to  60  miles]  in  length. 
Usually,  also,  one  of  the  edges  of  the  crevice  so 
formed  is  raised  with  respect  to  the  other  side, 
which  is  lower.  Often  there  is  a  displacement  of 
level  in  the  horizontal  sense,  and  if,  for  example, 
the  region  affected  by  the  earthquake  is  traversed 
by  a  road,  this  may  be  cut  in  such  a  way  that  the 
two  pieces  have  no  longer  either  the  same  direc- 
tion or  the  same  level.  Earthquakes  thus  give 
rise  to  permanent  deformations  of  the  crust,  de- 
formations the  origin  of  which  is  a  sudden  move- 
ment of  the  latter.  The  exact  surveys  carried 


224  The  Earth 

out  by  the  government  officials  of  the  different 
countries  have  demonstrated  permanent  differ- 
ences of  level  of  more  than  2  metres  [or  yards] 
in  the  regions  affected  by  the  more  important 
earthquakes,  such  for  example  as  that  established 
by  the  geodetic  operations  carried  out  in  Croatia 
after  the  Agram  earthquake  in  1885. 

But  in  addition  to  these  permanent  deformations 
of  sudden  origin,  there  are  also  slow  deformations 
that  our  Earth's  crust  undergoes  continuously. 
We  can  only  perceive  these  movements  by  an 
advance  or  recession  of  the  seashores,  which 
appear  to  encroach  on  the  land  or  move  seawards 
as  the  case  may  be.  Examples  of  such  occurrences 
are  abundant;  in  the  Red  Sea  may  be  seen  lines 
of  coral  reef  of  relatively  recent  date,  emerging 
above  the  actual  sea-level,  and  which  could  only 
have  been  constructed  by  their  microscopic 
builders  when  under  a  protective  layer  of  water, 
which  must  thus  have  covered  them  not  long  since. 
In  Scandinavia  a  kind  of  sea-saw  movement  is 
taking  place;  the  bottom  of  the  Gulf  of  Bothnia 
is  rising  while  the  southern  part  of  the  peninsula 
seems  to  be  gradually  sinking  into  the  sea. 

By  comparing  the  coast  reference  marks  traced, 
by  Celsius,  on  the  rocks  on  the  shores  of  Sweden, 
in  1730,  we  are  able  to  prove  ground  move- 


Seismic  Phenomena  225 

ments  reaching  nearly  2  metres  [or  yards]  per 
century,  and  similar  facts  have  been  verified  in 
Norway,  Finland,  and  Siberia.  Everyone  knows 
the  classical  alternation  of  risings  and  fallings 
exhibited  by  the  columns  of  the  Temple  of  Ser- 
apis  at  Pozzuoli,  a  movement  which  averages  I 
millimetre  [.039  inch]  per  year.  In  the  Indies, 
subterranean  forests  have  been  discovered; 
in  Prussia,  lakes  of  relatively  recent  formation 
exist  in  depressions  of  the  ground.  Finally,  in 
mountainous  countries,  a  spectator  placed  upon 
a  summit  may  see  distant  mountains  just  above 
a  near  hill  in  front  of  him;  now  in  several  cases 
such  visibility  of  the  distant  peaks  has  ceased, 
on  account  of  a  slight  upraising  of  the  interposed 
hill  or  of  a  sinking  of  the  distant  peak  in  question. 
These  phenomena  have  taken  place  in  French  Jura, 
in  Spain,  Bohemia,  Switzerland,  and  Thuringia. 
The  sinkings  of  the  constituent  portions  of  the 
Earth's  crust  are  thus  a  general  and  permanent 
phenomenon;  when  they  do  not  occur  suddenly, 
they  operate  slowly,  but  they  take  place  unceas- 
ingly, giving  a  perpetual  mobility  to  the  ground 
that  seems  to  us  so  firm.  From  this,  it  will  be 
recognised  how  great  is  the  importance,  from  the 
point  of  view  of  the  Earth's  history,  of  those 
precise  determinations  of  altitude,  which  are  the 
is 


226  The  Earth 

only  means  of  detecting  and  measuring  relative 
changes  of  altitude  of  different  points  on  the  solid 
surface  of  the  terrestrial  globe. 

There  is  one  further  manifestation  of  the  inte- 
rior activity  of  the  globe  which  may  lead  to  terrible 
catastrophes,  viz.,  the  violent  movements  of  the 
sea,  which  are  wrongly  called  tidal  waves,  for 
they  have  no  connection  with  the  periodic  pheno- 
mena of  flux  and  reflux  of  the  waters  of  the  sea. 

A  tidal  wave  originates  in  a  seismic  disturbance 
occurring  at  a  place  on  the  bottom  of  the  sea. 
Such  a  phenomenon  may  happen  as  a  result  either 
of  a  sudden  upraising  or  a  sudden  lowering  of  the 
submarine  surface.  Let  us  suppose,  for  example, 
that  the  cause  is  the  former;  a  liquid  protuberance 
immediately  forms  at  the  upper  surface  of  the 
sea  over  the  part  of  the  submarine  surface  raised. 
This  protuberance  is  bordered  by  two  hollows, 
the  depth  of  which  is  in  proportion  to  the  height 
of  the  uplifted  mass  of  water,  and  so  gives  rise 
to  a  wave  which  extends  outwards  over  the  sur- 
face of  the  ocean,  reproducing  on  a  vast  scale 
the  phenomenon  of  rings  which  is  caused  by  drop- 
ping a  stone  into  water.  A  wave  which  is  called 
the  seismic  wave  of  translation  is  thus  propagated 
over  the  surface  of  the  sea.  Its  arrival  at  the 
coast  is  preceded  by  a  lowering  of  the  water  to 


Seismic  PHenomena  227 

the  same  extent,  which  at  the  moment  of  reaching 
the  shore  produces  there  a  retreat  of  the  sea. 
Ships  are  suddenly  grounded  on  the  bottom  of 
the  ports  where  they  are  at  anchor.  But  an  instant 
later  the  high  wave  follows  this  retreat,  and  the 
vessels  are  sometimes  carried  a  considerable  dis- 
tance on  to  dry  land  by  the  crest  of  the  great  wave 
which  has  refloated  them.  Water  breaks  over 
low  coasts,  submerging  habitations  and  drowning 
men  and  animals  in  its  passage;  this  is  what  oc- 
curred at  Lisbon,  in  1755,  when  a  terrible  tidal 
wave  followed  a  great  earthquake.  Thirty  thou- 
sand persons  were  killed  by  these  two  causes 
combined. 

Tidal  waves  are  formidable  even  on  coasts  like 
those  of  Portugal,  but  are  far  more  so  in  the  case 
of  low  shores  such  as  exist  in  the  archipelagoes 
of  Polynesia.  This  is  especially  so  as  they  are 
situated  in  the  Pacific,  a  region  where  earthquakes 
are  numerous  and  therefore  are  more  exposed  to 
these  terrible  phenomena.  These  islands  have 
but  a  very  slight  elevation  above  sea-level  and 
when  a  tidal  wave  descends  upon  one  it  destroys 
everything  upon  it. 

The  velocity  of  propagation  of  these  waves  of 
seismic  origin  attains  considerable  values;  they 
may  move  over  the  surface  of  the  sea  at  the  rate 


228  The  Earth 

of  350  to  400  nautical  miles  an  hour.  The  marine 
mile  equals  1852  metres  and  is  the  length  of  one 
minute  of  arc  measured  on  a  terrestrial  meridian. 
Therefore  these  seismic  waves  of  translation  are 
propagated  with  the  velocity  of  750  to  800  kilo- 
metres [465  to  495  miles]  per  hour.  The  eruption 
of  Krakatoa  was  accompanied  by  a  violent  local 
seismic  phenomenon  and  this  produced  a  gigantic 
wave  of  translation,  which  made  itself  felt  two 
days  afterwards  by  the  tide  recorder  at  Rochefort. 
In  the  regions  of  Japan  or  Peru  earthquakes  fre- 
quently accompany  volcanic  eruptions,  and  on 
every  occasion  it  has  been  shown  that  the  wave 
traverses  the  entire  width  of  the  Pacific  in  twelve 
hours. 

Oceanographers  have  made  a  study  of  this  pro- 
pagation and  found  a  simple  law  governing  it. 
If  the  mean  depth  of  the  ocean  at  whose  surface 
the  wave  is  moving  be  multiplied  by  the  mean 
intensity  of  gravity  along  the  track  of  the  wave, 
the  square  root  of  the  product  is  equal  to  the 
velocity  of  propagation.  As  a  consequence  of  this 
simple  law  we  may  in  nearly  every  case  deduce 
the  mean  depth  of  the  ocean  over  whose  surface 
the  wave  passes.  For  we  can  usually  deter- 
mine the  velocity  of  propagation  of  an  important 
seismic  wave  of  translation,  since  the  earthquake 


Seismic   Phenomena  229 

giving  rise  to  it  occurs  at  a  known  hour,  and  the 
moment  the  wave  reaches  the  opposite  shore  of 
the  sea  or  ocean  also  can  be  precisely  found  by 
means  of  tide  recorders.  The  mean  intensity  of 
gravity  along  the  path  of  the  wave  is  also  known. 
A  remarkable  agreement  is  found  between  the 
results  of  direct  soundings  and  those  furnished 
in  this  manner,  constituting  a  beautiful  confirma- 
tion of  the  theory  of  the  propagation  of  the  waves. 


CHAPTER  VIII 

THE  MAGNETISM,  ELECTRICITY,  AND  RADIOACTIVITY 
OF  THE  EARTH 

PHE  preceding  chapters  have  shown  us  how 
*  far  the  terrestrial  globe  resembles  a  living 
being,  with  its  period  of  growth  and  development, 
its  internal  activity,  the  regular  pulsation  of  its 
crust,  and  the  convulsive  shocks  which  agitate  it 
at  intervals.  We  are  now  to  see  that  phenomena 
of  circulation  are  produced  in  the  terrestrial  crust 
under  the  form  of  electric  currents  with  associated 
magnetic  phenomena,  which  are  inseparable  from 
the  former.  The  likelihood  of  magnetic  pheno- 
mena is  obvious  from  what  we  have  previously 
learned  about  the  Earth's  nucleus.  By  reason  of 
the  enormous  pressures  to  which  its  various  parts 
are  subjected,  it  is  in  a  condition  practically  equi- 
valent to  the  solid  state,  in  spite  of  the  high  tem- 
perature, and  is  furthermore  constituted  of  metallic 
elements,  among  which  iron  predominates.  It  is 
thus  not  surprising  that  the  terrestrial  globe  in  its 
entirety  should  exhibit  magnetic  properties. 

230 


Electric  and  Magnetic  PKenomena   231 

On  the  other  hand,  the  Sun  possesses  a  consid- 
erable electrostatic  charge  and  therefore  creates 
an  electric  field  about  itself.  The  physicist  Nodon 
was  undoubtedly  the  discoverer  of  the  electric 
action  of  the  solar  rays;  in  1885,  he  proved  that 
the  Sun's  radiation  produced  a  positive  charge  in 
an  insulated  conductor,  a  charge  which  increased 
with  the  intensity  of  the  radiation,  the  pheno- 
menon ceasing  when  clouds  passed  before  the  Sun. 
In  1905,  Bernard  Brunhes  confirmed  Nodon 's 
results  by  a  very  beautiful  series  of  experiments, 
and  Nodon  himself,  at  the  Pic  du  Midi  in  1907, 
clearly  demonstrated  the  solar  electrical  action. 
Our  Sun  is  therefore  charged  with  electricity. 
This  charge  doubtless  surpasses  in  magnitude  any 
imaginable  one;  Arrhenius's  calculations  show 
that  it  is  always  as  great  as  250  thousand  million 
coulombs.1  It  produces  an  electrostatic  field. 

The  charge,  turning  rapidly  around  the  Sun's 
axis  of  rotation,  must  give  rise  to  a  magnetic  field, 
this  latter  being  a  consequence  of  the  motion  of 
a  charge,  as  was  discovered  by  Rowland.  This 
phenomenon  was  disputed  for  a  long  time,  but 
the  experiments  of  the  Roumanian  physicist 

1 A  coulomb  "is  the  quantity  due  to  the  passage  of  a  current 
of  i  ampere  for  i  second,  or  of  %  ampere  for  2  seconds,  and  so 
on  "  (Ganot's  Physics).— Ed. 


232  The  Earth 

Vasilesco-Karpen  have  definitely  verified  its  ex- 
istence and  measured  it  quantitatively.  The 
intensity  of  the  field  so  created  is  doubtless  con- 
siderable in  the  neighbourhood  of  the  solar  sur- 
face but  certainly  feeble  at  a  distance  such  as  that 
of  the  Earth  from  the  Sun,  for  the  law  of  decrease 
of  magnetic  action  is  that  it  is  inversely  as  the 
cube  of  the  distance.  Nevertheless,  even  if 
feeble,  the  field  undoubtedly  exists,  and  the  con- 
ducting nucleus  of  the  Earth  moves  through  it 
with  great  velocity  like  the  armature  coils  of  a 
dynamo  in  the  field  of  its  field-magnets.  There 
must,  thus,  be  produced  "Foucault's  currents" 
which  flow  through  the  terrestrial  mass. 

This  is  not  all,  however ;  the  positively  electrified 
surface  of  the  Sun  sends  into  space  small  negatively 
charged  particles;  as  we  have  previously  seen,  this 
repulsive  effect  being  superior  to  the  attractive 
force  in  the  case  of  very  small  particles,  because 
the  pressure  of  radiation  is  relatively  greater 
upon  them.  Many  of  these  minute  fragments 
reach  the  Earth's  atmosphere.  The  effect  of 
ultra-violet  light,  as  the  work  of  Lenard  has  shown, 
is  to  discharge  these  particles,  and  it  is  their 
negative  charge  which  escapes  in  the  form  of  what 
are  called  electrons  in  the  language  of  modern 
physics.  These  electrons  are  excessively  small, 


Electric  and  Magnetic  PHenomena   233 

and  some  idea  of  them  may  be  obtained  from  the 
fact  that  a  thousand  of  them  weigh  almost  as 
much  as  one  atom  of  hydrogen,  while  one  gram 
[15.4  gr.]  of  hydrogen  contains  a  number  of  atoms 
represented  by  the  figure  I  followed  by  twenty- 
four  zeros. 

By  the  well-known  demonstration  of  iron  filings 
arranging  themselves  along  the  magnetic  lines  of 
force,  we  know  that  these  lines  of  force  converge 
to  the  poles  of  a  magnet  (Figs.  23  and  24).  Now, 
the  rays  of  the  solar  corona  have  a  deflection 
towards  the  equator  from  the  poles  similar  to  the 
lines  of  the  magnetic  figures.  We  may  therefore 
suppose  that  the  Sun  behaves  like  a  great  magnet, 
the  magnetic  poles  of  which  practically  coincide 
with  the  geometrical  poles. 

Electrified  particles  shot  forth  from  the  Sun 
may  reach  the  Earth,  and,  in  the  course  of  the 
present  chapter,  we  shall  see  beautiful  experimen- 
tal verifications  of  this  theoretical  conception. 
These  particles  convey  their  charges,  the  influence 
of  which  is  felt  in  the  atmosphere  and  at  the  sur- 
face of  the  ground.  The  ions  of  the  upper  atmo- 
sphere partake  of  the  Earth's  movement;  they 
have  been  repelled  by  the  Earth,  which  is  simi- 
larly electrified,  and  remain  in  the  higher  layers 
of  our  atmosphere,  where  they  produce  electrical 


234 


The  Earth 


phenomena   which   have   a   powerful   generative 
or  modifying  action  on  the  terrestrial  magnetism. 


FIG.  23. — Magnetic  Lines  of  Force  (Two  Poles  of  Opposite 
Sign),  after  Prof.  Stanotevitch. 

To  recapitulate,  the  Earth  moves  in  an  electric 
and  magnetic  field  due  to  the  Sun,  and  receives 


FTG.  24. — Magnetic  Lines  of  Force  (the  Two  Poles  of  a  Cir- 
cular Magnet  after  Prof.  StanoieVitch. 

from  the  Sun  particles  which  bring  charges  to  its 
surface.     Any  change  in  the  intensity  of  the  solar 


Electric  and  Magnetic  PHenomena   235 

radiation  will  modify  the  intensity  of  the  observed 
effects ;  also  any  change  in  the  velocity  of  displace- 
ment of  the  Earth  in  this  field  will  cause  the  in- 
direct effects  to  vary.  Now,  Kepler's  law  tells 
us  that  such  modifications  of  velocity  do  occur, 
and  we  may  therefore  expect  periodical  variations 
of  electric  and  magnetic  phenomena.  Such  being 
the  theoretical  considerations,  we  shall  now 
examine  the  results  of  actual  observations. 

Terrestrial  magnetism,  with  which  we  shall 
commence,  shows  itself  in  a  simple  way  by  the 
directive  effect  which  the  Earth  has  on  a  magnet- 
ised needle  freely  suspended  at  its  centre  of  gravity. 
This  elementary  experiment,  more  or  less  modified, 
is  the  basis  of  all  our  study  of  the  manifestations 
of  the  Earth's  magnetism. 

The  vertical  plane  which  contains  the  needle 
is  called  the  magnetic  meridian.  It  points  nearly 
towards  the  north  pole  of  the  Earth,  but  in  general 
does  not  coincide  with  the  geographical  meridian 
of  the  place,  the  angle  between  the  two  being 
called  the  declination.  The  angle  made  with  the 
horizontal  by  the  needle  is  known  as  the  inclina- 
tion, and  the  intensity  of  the  horizontal  component 
of  the  force  directing  the  needle,  which  is  an  im- 
portant quantity,  is  often  called  the  horizontal 
intensity. 


236  The  Earth 

If  a  magnetic  needle  be  suspended  and  means 
are  furnished  for  the  measurement  of  its  direction 
with  all  possible  precision,  it  may  be  proved  ex- 
perimentally that,  in  any  one  place,  the  magnetic 
elements  vary  during  each  day.  These  variations 
recur  regularly  every  day  in  the  year,  and  their 
mean  values  also  vary  according  to  the  season. 
As  regards  the  declination,  which  is  actually  10° 
£o  the  west  at  Paris  (1912),  this  quantity  passes 
daily  through  a  maximum  and  minimum  value, 
but  the  actual  degree  of  variation  is  slight, 
attaining  only  a  few  minutes  of  arc.  The  inclina- 
tion and  the  horizontal  intensity  undergo  ana- 
logous variations,  but  these  differ  in  sign  in  the 
respective  hemispheres.  The  variations  are  greater 
in  warm  weather  than  in  cold  weather. 

The  declination  shows  annual  periodical  varia- 
tions such  as  the  theory  indicates;  every  year 
there  is  a  maximum  and  a  minimum,  the  changes 
of  sign,  that  is  to  say  the  passages  through  the 
mean  value,  taking  place  about  the  time  of 
the  equinoxes.  These  epochs  also  determine  the 
change  of  sign  of  the  variations  of  the  inclination 
and  the  horizontal  intensity. 

No  relation  between  the  lunar  period  and  the 
variation  of  the  magnetic  elements  has  yet  been 
established  with  certainty.  It  is  the  contrary 


Electric  and  Magnetic  Phenomena   237 

with  regard  to  the  solar  spots  and  we  here  find  one 
of  the  most  beautiful  confirmations  of  the  theo- 
retical views  which  we  have  enunciated  at  the  be- 
ginning of  this  chapter.  The  years  of  maximum 
solar  spots  are  those  in  which  the  variation  of  the 
declination  and  that  of  the  horizontal  intensity 
also  attain  their  maxima,  and  the  curves  which 
represent  these  three  phenomena  more  or  less 
coincide  with  one  another. 

Thus,  at  any  one  place,  the  declination,  in 
common  with  the  other  magnetic  elements,  under- 
goes variations  of  daily  and  annual  periodicity, 
and  also  one  of  nj  years'  period,  and  these  all 
correspond  to  the  known  periods  of  the  varia- 
tions of  the  effects  resulting  from  the  solar  ac- 
tivity, whether  on  account  of  the  varying  amounts 
of  heat  received  by  the  Earth,  or  by  reason  of 
the  variation  in  the  distance  between  the  two 
bodies,  or  finally  because  of  the  change  in  the 
radiation  emitted  by  the  Sun. 

But  this  is  not  all ;  if  the  declination  be  carefully 
observed,  and  if  its  mean  value  be  taken  for  each 
year,  it  may  be  proved  that  it  varies  showly  from 
one  year  to  another  and  that  this  secular  variation 
appears  to  be  also  periodic.  At  Paris,  for  example, 
observations  of  the  declination  have  been  made 
since  the  year  1540.  The  phenomenon  itself  was 


The  Earth 


1500 


1600 


1700 


1800 


1900 


CAST 


discovered  by  Christopher  Columbus,  in  1492, 
at  the  time  of  his  voyage  resulting  in  the  discovery 
of  America.  Now,  in  1540,  the  declination  at 
Paris  was  to  the  east,  that  is  to  say  the  magnet- 
ised needle  pointed  to  the  eastward  of  the  geo- 
graphical north  (Fig.  25) ;  its  value  subsequently 
increased  and  passed  through  a  maximum  value 

a    little    before 
the    year    1600. 
Then    it    began 
to  decrease  and 
reached  zero  in 
1660,   in    which 
FIG.  25. — Secular  Variations  of  the  De-   year    compasses 
dination  at  Paris.  indicated   the 

true  north  in  the  French  capital.  After  1660,  the 
declination  changed  sign  and  became  westwards, 
increasing  in  value  each  year.  At  the  beginning 
of  the  nineteenth  century,  it  attained  a  maximum 
of  about  24°  since  when,  though  remaining  west- 
wards, it  has  continually  diminished.  Actually 
it  is  about  10°  to  the  west  and  is  thus  approach- 
ing the  zero  value  again,  when  it  will  change  sign 
and  pass  to  the  east,  where  it  was  before  1660. 

As  regards  the  inclination,  the  precise  results 
of  direct  observations  are  less  decisive,  and  its 
importance,  for  sailors  and  travellers,  is  much  less 


Electric  and  Magnetic  PHenomena   239 

than  that  of  the  declination,  which  gives  them  a 
fixed  direction  when  the  stars  are  hidden  by  cloud. 
But,  in  the  course  of  recent  years,  Giuseppe 
Folgheraiter  has  thrown  a  great  light  on  the  matter 
by  some  very  original  work. 

Potter's  clay  is  magnetic;  a  vase  moulded  from 
such  clay  becomes  therefore  magnetised  by  in- 
duction under  the  influence  of  the  terrestrial 
field,  that  is  to  say  it  exhibits  two  poles  so  placed 
that  the  line  which  would  join  them  is  parallel 
to  the  direction  of  the  dip  needle.  Thus  the  vase 
behaves  just  as  a  piece  of  soft  iron  would.  But, 
if  while  thus  subjected  to  the  action  of  the  Earth's 
magnetic  field,  the  clay  vessel  is  placed  in  an 
oven  and  baked,  its  magnetisation  will  become 
permanent  and  the  two  poles,  situated  on  a  line 
parallel  to  the  direction  of  the  dip  needle,  will 
remain  permanently.  If,  therefore,  we  knew  in 
what  way  the  vase  was  oriented  in  the  oven  with 
respect  to  the  geographical  north  we  could  deduce, 
by  investigation,  its  permanent  magnetism,  both 
the  direction  of  the  magnetic  meridian  and  the 
value  of  the  inclination  at  the  time  when  it  was 
baked. 

The  researches  of  Folgheraiter  were  carried 
out  on  Etruscan  vases,  the  antiquity  of  which  is 
considerable.  These  vases,  having  the  forms  of 


240  The  Earth 

surfaces  of  revolution,  give  no  exterior  indication 
by  which  we  can  arrive  at  their  orientation  in  the 
oven  at  the  time  of  their  manufacture ;  they  cannot 
thus  furnish  us  with  any  information  as  to  the 
declination.  But  as  regards  the  inclination,  which 
is  the  angle  made  by  the  magnetic  needle  with 
the  horizontal,  they  give  us  a  sufficiently  exact 
value,  for  they  were  always  placed  on  a  horizontal 
plane  in  the  baking  oven  and,  whatever  subsequent 
positions  they  were  placed  in,  the  angle  made  by 
their  poles  with  the  horizon  can  be  determined 
by  again  replacing  them  in  a  horizontal  position. 

A  very  remarkable  result  was  deduced  and  it 
was  at  first  very  much  disputed,  viz.,  that  the 
magnetic  inclination  must  have  been  zero,  in 
Central  Italy,  towards  the  middle  of  the  sixth 
century  B.C.,  and  that  in  the  preceding  years, 
that  is  to  say  in  the  course  of  the  seventh  century 
B.C.,  several  specimens  of  the  art  of  which  remain 
to  us,  the  north  pole  of  the  magnetic  needle  was 
inclined  above  the  horizontal  instead  of  below  it 
as  at  the  present  time. 

Bernard  Brunhes,  in  1906,  was  enabled  to  con- 
firm these  conclusions  of  Folgheraiter  by  means  of 
investigations  he  carried  out  on  the  permanent 
magnetisation  of  the  lavas  of  the  Puy-de-D6me. 
His  work  dealt  with  the  metamorphic  clay  of  the 


Electric  and  Magnetic  Phenomena  241 

lava  of  Pontfarein,  in  Cantal;  in  contact  with  the 
burning  lava  this  clay  is  baked  in  situ  as  if  it  had 
been  in  a  pottery  oven.  Brunhes  has  deduced 
clear  indications  of  a  change  of  sign  of  the  incli- 
nation having  occurred  in  these  regions  at  an 
epoch  not  very  different  from  that  which  Folghe- 
raiter  indicated. 

Brunhes  has,  however,  done  yet  more.  He  has 
made  a  study  of  the  pavement  of  the  Temple  of 
Mercury  erected  on  the  summit  of  the  Puy-de- 
D6me  and  has  justified  the  principle  of  the 
work  of  the  Italian  scientist.  The  paving  stones 
which  form  the  floor  of  the  temple  are  constituted 
of  volcanic  rocks  and  each  one  has  retained  a 
permanent  magnetisation.  The  decimation  de- 
duced varies  from  one  stone  to  another,  as  would 
be  expected,  since  they  are  oriented  in  different 
ways,  but  the  inclination  is  the  same  for  all;  the 
elements  of  their  magnetisation,  which  date  from 
the  time  of  their  cutting,  have  therefore  not  been 
affected  by  the  subsequent  variations  of  the  ter- 
restrial magnetism.  Consequently  the  conclusions 
of  Folgheraiter  on  the  ancient  values  of  the  incli- 
nation, deduced  from  the  study  of  the  Etruscan 
vases,  are  perfectly  legitimate. 

Thus  the  magnetic  elements  not  only  vary  in 
the  course  of  each  day,  each  year,  and  each  eleven- 

16 


242  The  Earth 

year  solar  period,  but  also  suffer  slow  variations 
in  the  course  of  successive  centuries.  Here  we 
again  realise  that  there  is  a  perpetual  evolution 
in  those  forces,  the  play  of  which  constitutes  the 
life  of  our  Earth.  We  shall  now  see  that  these 
variations  with  time  are  not  the  only  ones,  but 
that  there  are  also  variations  according  to  position 
on  the  Earth's  surface. 

A  magnetised  steel  needle  freely  suspended 
about  its  centre  of  gravity  takes  up  a  position 
inclined  to  the  horizontal.  By  applying  a  light 
counterpoise  to  the  higher  end,  we  are  able  to 
force  it  into  the  horizontal  position  and  we  may 
then  prove  that  it  will  always  direct  itself  towards 
a  point  on  the  horizon  called  the  magnetic  north, 
being  free  to  move  in  the  horizontal  plane  in 
which  it  is  constrained  to  remain. 

If  we  move  over  the  Earth's  surface,  walking 
always  in  the  direction  of  a  horizontal  magnetised 
needle,  that  is,  a  declination  needle,  we  shall  go 
towards  this  special  north,  and  our  journey  will 
be,  not  along  a  terrestrial  meridian,  but  over  a 
curved  line  which  is  called  a  magnetic  meridian. 
All  such  magnetic  meridians  converge  towards 
a  point  situated  in  Northern  Canada,  to  which 
the  name  of  the  North  Magnetic  Pole  has  been 
given.  In  the  southern  hemisphere,  there  is  a 


Electric  and  Magnetic  Phenomena   243 

South  Magnetic  Pole,  situated  in  Victoria  Land, 
part  of  the  Antarctic  continent  not  far  from  the 
volcanic  mountains  Erebus  and  Terror.  The  North 
Magnetic  Pole  has  been  several  times  reached 
by  explorers,  the  latest  being  the  Danish  explorer 
Roald  Amundsen  some  years  ago;  as  regards  the 
South  Magnetic  Pole,  Sir  Ernest  Shackleton  deter- 
mined its  position  in  1910.  It  should  be  noted  that 
at  either  of  these  poles  a  magnetised  declination 
needle  will  lie  equally  in  any  direction  and  not 
only  in  one  fixed  one,  whereas  the  inclination 
needle  is  vertical.  It  is  this  latter  which  enables 
us  to  determine  the  situation  of  the  magnetic 
pole. 

The  magnetic  poles  are  not  fixed  at  the  Earth's 
surface.  They  are  distant  from  the  geographical 
poles,  but  the  North  Magnetic  Pole,  although 
incessantly  moving,  never  wanders  far  from  lati- 
tude 69°  north,  while  the  mean  latitude  of  the 
South  Magnetic  Pole  is  75°.  Between  1770  and 
1888  the  North  Magnetic  Pole  moved  from  lati- 
tude 66°  to  latitude  71°;  it  has  reached  a  point 
more  than  600  kilometres  [372  miles]  nearer  the 
terrestrial  pole  than  at  the  earlier  date.  It  now 
appears  to  be  retreating  again.  This  non-fixity 
of  the  magnetic  poles,  the  incessant  fluctuation 
of  their  position,  corresponds  to  the  secular  varia- 


244  The  Earth 

tion  of  the  elements  of  terrestrial  magnetism, 
and  one  of  the  phenomena  is  a  direct  consequence 
of  the  other. 

In  order  to  go  from  one  point  on  the  Earth  to 
the  magnetic  pole  we  have  only  to  follow  a  route 
always  tangential  to  the  direction  of  the  inclina- 
tion needle.  For  every  place  on  the  Earth  the 
value  of  the  declination  may  be  measured,  that 
is  to  say  the  angle  between  the  directions  of  the 
needle  and  the  geographical  meridian  may  be 
found.  By  such  means  it  can  be  shown  that  the 
declination  varies  from  one  point  to  another  of  the 
globe.  It  is  a  matter  of  the  greatest  importance, 
for  sailors  and  travellers,  to  know  these  variations 
for  the  different  parts  of  the  Earth,  since  when 
they  are  unable  to  observe  the  stars  in  order  to 
deduce  the  position  they  are  in  at  any  time  they 
can  only  direct  themselves  by  means  of  the  magne- 
tic needle.  Consequently  it  is  essential  to  know 
the  difference  between  the  magnetic  north  and 
the  true  north  at  a  given  place,  and  how  this 
difference  varies  from  point  to  point  of  the  terres- 
trial surface.  Magnetic  maps  of  the  Earth  have 
been  drawn  up  by  tracing  on  a  planisphere  lines 
passing  through  points  on  the  Earth's  surface 
where  the  declination  has  the  same  value;  these 
lines  are  called  isogonic  lines. 


Electric  and  Magnetic  PKenomena   245 

It  is  obvious  that  all  the  isogonic  lines  must 
pass  through  the  magnetic  poles.  They  also 
pass  through  the  geographical  poles,  since  the 
declination  is  the  angle  between  the  magnetic 
meridian  and  the  geographical  meridian.  As  all 


FIG.  26. — Chart  of  Isogonic  Lines  or  Lines  of  Equal  Magnetic 
Declination. 


geographical  meridians  pass  through  the  poles 
of  the  Earth  the  declination  there  can  have  any 
value,  so  that  the  isogonic  lines  must  all  meet 
together  there.  Fig.  26  gives  an  idea  of  such  a 
map.  Similar  curves  have  been  drawn  for  the 
inclination  and  the  horizontal  intensity,  but  the 
importance  of  the  declination  for  navigation  and 
land  journeys  makes  the  isogonic  maps  of  more 
immediate  interest. 


246  The  Earth 

There  are  certain  peculiarities  which  strike  one 
at  once  upon  this  map,  for  example,  there  are 
three  lines  of  zero  declination,  drawn  thicker  than 
the  rest. 

Between  the  two  chief  thicker  lines  passing 
through  points  on  the  Earth  where  the  declina- 
tion is  zero  are  found  regions  where  it  is  west; 
outside  them  it  is  east.  Nevertheless  there  are 
two  portions  of  the  Earth's  surface  characterised 
by  singularities.  There  is  a  closed  curve  on 
Eastern  Siberia,  along  which  the  declination  is 
zero,  and  in  its  interior  the  declination  again 
becomes  west.  Also  in  the  Eastern  Pacific  there 
is  another  closed  curve  corresponding  to  a  mini- 
mum declination.  It  will  be  seen  from  these  re- 
marks how  curious  the  distribution  of  terrestrial 
magnetism  is. 

As  the  magnetic  elements  vary  with  time  and 
also  according  to  the  region  of  the  Earth  consid- 
ered, it  is  obvious  that  magnetic  maps  should  be 
frequently  remade  to  correspond  with  the  new 
values  of  the  elements,  so  that  travellers  should 
have  correct  data  and  not  erroneous  ones  which 
might  lead  them  wrongly,  and  even  into  danger. 

The  variations  of  the  magnetic  elements  of 
which  we  have  spoken  up  to  the  present  have  been 
the  slow  and  continuous  ones;  there  are  others 


Electric  and  Magnetic  Phenomena   247 

of  a  sudden  character  which  constitute  magnetic 
storms  and  perturbations. 

When  magnetic  instruments  of  great  precision 
are  installed  in  an  observatory,  enabling  us  to 
indicate,  and  preserve  by  photographic  registra- 
tion, the  least  variation  in  terrestrial  magnetism, 
we  ordinarily  observe  the  periodic  variations  that 
have  been  described  above.  But  on  certain  days 
the  needles  are  agitated ;  they  tremble  and  exhibit 
quite  erratic  movements,  their  oscillations  obeying 
no  regular  law.  Often  in  such  cases  these  irregu- 
larities and  agitations  of  the  magnetic  needle  are 
great  enough  to  be  observed  in  ordinary  compasses. 
Such  a  phenomenon  is  called  a  perturbation  or 
magnetic  storm. 

A  magnetic  storm  always  makes  itself  felt  over 
a  considerable  portion  of  the  Earth's  surface,  and 
very  frequently  its  occurrence  coincides  with 
polar  aurorae  and  with  important  seismic  pheno- 
mena. We  have  seen  that  it  is  possible  to  conceive 
how  the  movements  of  the  internal  nucleus  may 
affect  the  magnetism  and  produce  disturbance 
of  the  crust,  so  that  it  is  not  surprising  that  these 
phenomena  exhibit  a  certain  degree  of  coincidence. 
We  shall  see  later  on  why  polar  aurorae  often  mani- 
fest themselves  at  the  same  time  as  magnetic 
storms.  It  is  an  incontestable  fact,  the  result 


248  THe  Earth 

of  actual  observation  and  not  only  of  theory,  that 
the  forms  of  the  curves  representing  respectively 
the  periodicities  of  the  solar  spots,  the  aurorae 
boreales,  and  magnetic  storms  are  identical;  the 
three  curves  have  exactly  the  same  aspect  and  the 
same  irregularities. 

Independent  of  the  general  variations  which 
the  Earth's  magnetic  elements  undergo  as  we 
pass  from  one  point  of  the  surface  to  another, 
local  anomalies  may  be  observed  exactly  analogous 
to  the  case  of  the  value  of  the  intensity  of  gravity, 
where  we  observe  local  irregularities  arising  from 
particular  local  effects  at  the  given  point  in  ques- 
tion. The  crust  being  of  varying  thickness,  the 
surface  is  consequently  unequally  distant  in 
different  parts  from  the  central  nucleus  containing 
the  metallic  elements  to  which  the  earth's  mag- 
netism is  due.  Furthermore,  as  the  crust  itself 
may  contain  more  or  less  magnetic  mineral  matter, 
we  may  readily  understand  how  purely  local 
variations  may  arise  from  both  these  causes,  viz., 
an  exceptional  thickness  or  thinness  of  the  crust 
at  the  place  under  consideration  and  its  geological 
nature. 

This  general  explanation,  while  it  is  doubtless 
sufficient  in  many  cases,  is,  however,  far  from  satis- 
factory in  others.  Thus,  in  the  region  of  Paris, 


Electric  and  Magnetic  PHenomena   249 

there  exists  a  very  marked  local  anomaly;  the 
isogonic  lines  are  folded  on  themselves  in  the  form 
of  an  S  with  very  serrated  bends.  Now  it  is  not 
possible  to  find  a  magnetic  cause  of  this  anomaly 
in  the  geology  of  the  Parisian  region;  the  strata 
are,  in  fact,  chalk.  The  question  arises  whether 
the  cause  of  the  anomaly  is  to  be  sought  for  in  the 
deeper  strata.  The  S-shaped  curve  formed  by 
the  isogonic  lines  seems  to  be  the  continuation  of 
a  great  fault  in  the  district  of  Bray,  and  possibly 
this  fault,  by  reason  of  the  resulting  geological 
modification,  affects  the  circulation  of  the  electric 
currents  which,  as  we  shall  shortly  see,  incessantly 
traverse  the  terrestrial  crust.  Another  suggestion 
is  that  as  the  Tertiary  Parisian  basin  was  in  some 
measure  a  marine  formation,  the  ocean  which 
formerly  existed  there  corresponded  to  a  thinner 
crust,  according  to  the  theory  of  Lippmann,  and 
consequently  the  magnetic  interior  of  the  Earth 
is  relatively  near  the  surface  of  the  ground  in 
that  region.  The  matter  has  not  yet  been  fully 
elucidated. 

Magnetic  phenomena  are  not  the  only  ones  that 
indicate  the  Sun's  influence  upon  the  Earth;  there 
are  electric  phenomena  which  manifest  themselves 
in  various  ways  around  us;  the  first  and  the  most 
important,  from  a  practical  point  of  view,  is  the 


250  The  Earth 

existence  of  earth  currents.  In  the  early  days  of 
the  electric  telegraph  the  physicist  Matteucci 
showed  that,  at  times,  the  telegraph  lines  indicated 
grave  disturbances,  and  he  remarked  and  called 
attention  to  the  coincidence  of  these  perturbations 
with  magnetic  storms  and  the  appearance  of  polar 
aurorae. 

At  the  present  time,  the  phenomenon  is  better 
known,  and  telegraph  lines  are  the  best  possible 
instruments  for  its  study.  It  consists  of  the  pas- 
sage of  currents  quite  different  to  those  which 
circulate  normally  in  the  wires.  These,  being 
superimposed  on  the  currents  transmitting  the 
messages,  confuse  the  latter  and  produce  signals 
which  are  unconnected  with  those  despatched 
along  the  wires. 

These  earth  currents  make  the  bells  ring  and 
sometimes  even  cause  a  spark  to  pass  between 
different  parts  of  the  receiving  apparatus.  The 
electromotive  force  of  these  currents  is  sometimes 
nearly  looo  volts,  the  lines  they  traverse  being 
several  hundred  kilometres  [or  miles]  in  length. 
By  utilising  for  their  study  the  telegraphic  line 
which  connects  Clermont-Ferrand  with  the  sum- 
mit of  the  Puy-de-D6me,  Bernard  Brunhes  has 
clearly  proved  the  influence  of  the  inclination  of 
the  line  on  the  degree  of  their  manifestations,  which 


,  Electric  and  Magnetic  PHenomena   251 

seem  to  have  little  connection  with  atmospheric 
phenomena,  but  appear  on  the  contrary  to  corre- 
spond closely  with  magnetic  phenomena.  Like 
the  latter,  the  earth  currents  follow  a  regular 
periodicity  and  thus  show  recurrent  variations, 
but  the  chief  perturbations  have  an  accidental 
character,  and  almost  always  coincide  with  the 
appearance  of  polar  auroras,  also  with  magnetic 
storms  and  with  important  seismic  disturbances. 
At  the  beginning  of  November,  1903,  telegraphic 
perturbation  of  the  kind  caused  by  earth  currents 
took  place,  producing  an  almost  complete  inter- 
ruption for  two  days  of  the  service  in  Western 
Europe.  This  very  intense  manifestation  of  the 
special  activity  of  the  earth  currents  coincided 
exactly  with  an  aurora  borealis,  with  a  magnetic 
storm  of  exceptional  intensity,  and  with  an  earth- 
quake which  destroyed  the  town  of  Turchiz  in 
Persia  on  November  1st.  Furthermore,  it  is 
remarkable  that,  at  the  same  time,  a  spot  of 
extraordinary  dimensions  made  its  appearance 
on  the  Sun's  surface.  In  order  to  render  complete 
our  account  of  terrestrial  electric  currents,  it 
should  be  added  that  there  are  also  currents  be- 
tween the  ground  and  the  atmosphere;  a  positive 
current  appears  to  flow  upwards  from  places  of 
mean  latitude  and  to  be  transmitted  by  the  upper 


252  The  EartK 

atmospheric  layers,  returning  to  the  ground  in  the 
neighbourhood  of  the  equatorial  regions,  the  cir- 
cuit being  completed  by  the  current  traversing 
the  ground  from  south  to  north.  So  there  is  an 
effect  analogous  to  the  true  earth  currents,  and 
one  which  in  certain  cases  is  probably  superimposed 
on  them. 

Thus,  we  have  the  fact  that  these  electric  mani- 
festations, the  earth  currents,  bear  some  direct 
relation  to  the  solar  activity,  and,  consequently, 
with  all  the  phenomena  that  depend  on  the  latter, 
which  we  have  noted  at  the  beginning  of  this 
chapter.  This  relation  is  another  confirmation 
of  those  theoretical  conceptions  which  trace  to 
the  solar  energy  and  its  fluctuations  all  the  very 
varied  manifestations  of  energy  observable  at  the 
Earth's  surface.  But  it  is  not  only  the  action  of 
the  solar  radiation  that  produces  electric  pheno- 
mena at  the  surface  of  our  globe;  another  cause 
is  to  be  sought  in  the  dust  particles  repelled  from 
the  Sun,  if  sufficiently  small,  by  the  pressure  of 
its  radiation,  and  consequently  driven  forth  into 
space.  These  particles  give  rise  to  another  class 
of  electric  phenomena  which  occur  in  the  terrestrial 
atmosphere. 

The  Sun,  being  in  a  magnetic  condition,  presents 
two  magnetic  poles,  just  as  the  Earth  itself  does. 


Electric  and  Magnetic  PHenomena   253 

The  Solar  Corona,  according  to  the  beautiful  theory 
of  Arrhenius,  is  composed  of  very  minute  particles 
which  the  pressure  of  radiation  has  driven  far 
away  from  the  Sun's  surface.  The  coronal  stream- 
ers, formed  by  these  particles  which  come  from 
the  region  of  the  Sun  near  the  poles,  are  deflected 
under  the  influence  of  the  magnetic  lines  of  force 
emanating  from  these  poles,  which  act  upon  the 
negatively  electrified  particles.  On  a  large  scale, 
it  is  exactly  the  same  as  the  elementary  physical 
experiment  of  the  magnetic  figures  obtained  with 
iron  filings,  demonstrating  both  the  existence  of 
the  lines  of  force  and  the  direction  of  the 'field. 
Under  the  repulsive  action  of  the  pressure  of  the 
Sun's  radiation  part  of  this  solar  dust  arrives  in 
the  neighbourhood  of  the  Earth.  As  the  latter 
is  magnetic  and  has  two  poles,  it  exerts  an  influ- 
ence upon  the  particles.  Consequently  these 
become  grouped  into  two  streams  which  are 
directed  towards  the  magnetic  poles  of  the  Earth, 
and,  as  in  all  probability  the  magnetic  poles  do 
not  crop  out  at  the  surface  of  the  ground,  but  are 
situated  at  some  depth  in  the  Earth's  interior, 
these  attracted  streams  are  simply  drawn  towards 
a  region  of  roughly  circular  form  surrounding  the 
terrestrial  magnetic  poles.  When  the  multitude 
of  arriving  particles  is  more  abundant  than  usual, 


254  The  Earth 

owing  to  exceptional  solar  activity  at  the  time, 
their  electrification  is  bound  to  affect  the  Earth's 
magnetism. 

When  the  dust  particles  enter  the  atmosphere, 
and  so  meet  the  molecules  of  air,  they  produce  a 
phosphorescent  glow  exactly  as  if  this  air  was 
subjected  to  the  action  of  electric  radiation  arising 
from  a  piece  of  some  radioactive  substance.  In 
other  words,  the  negatively  electrified  particles 
driven  from  the  Sun  are  discharged  on  entering 
the  upper  regions  of  the  Earth's  atmosphere  and 
emit  cathode  rays,  to  which  the  polar  aurorae  are 
due.  Professor  Birkeland  has  attempted  an  experi- 
mental study  of  the  particular  circumstances  of 
the  origin  of  the  aurora,  by  means  of  laboratory 
researches.  He  took  a  sphere  of  magnetised  steel, 
representing  the  Earth,  covered  with  a  fluorescent 
coating.  He  then  exposed  the  sphere  to  the  action 
of  cathode  rays,  the  point  of  contact  of  which 
with  the  sphere  was  shown  by  the  illumination  of 
the  fluorescent  coating  produced.  Thus,  he  arti- 
ficially reproduced  luminous  phenomena  resem- 
bling polar  aurorae,  and  as  he  used  the  cathode 
rays,  which  are  now  considered  to  consist  of  small 
negatively  charged  particles  moving  with  con- 
siderable velocity,  just  like  the  solar  dust  above 
described,  the  experiment  affords  a  beautiful 


Electric  and  Magnetic  Phenomena   255 

confirmation  of  the  theory  of  auroras  that  has 
just  been  briefly  given. 

Another  confirmation  has  been  furnished  by 
the  remarkable  observations  of  the  Italian  astro- 
nomer Ricco.  If  the  theory  is  exact  and  if  auroras, 
and  the  electric  phenomena  of  which  the  Earth 
is  the  seat,  have  their  origin  in  the  dust  particles 
expelled  from  the  Sun's  surface,  we  should  expect 
more  important  manifestations  at  the  epochs  of 
greatest  eruptive  solar  activity.  These  epochs 
are  those  at  which  the  solar  faculas  are  most 
developed,  at  which  periods  the  sun-spots  also  are 
largest  and  most  frequent.  In  a  word,  the  maxima 
and  minima  of  auroras  and  of  magnetic  perturba- 
tions should  coincide  with  those  of  the  Sun's 
activity,  and  observation  has  already  shown  that 
such  is  the  case.  Now,  if  the  cause  of  auroras 
is  to  be  found  in  the  contact  of  solar  dust  with  the 
Earth's  atmosphere,  this  dust,  which  is  material 
substance,  cannot  be  transmitted  through  space 
with  an  infinite  velocity,  but  on  the  contrary  must 
occupy  a  certain  time  in  reaching  us  from  the  Sun. 

It  is  actually  possible  to  calculate  this  velocity. 
Let  us  consider  a  very  small  non-transparent 
particle  .00016  millimetre  [i  mm.  =  .03937  in.]  in 
diameter,  a  size  which  corresponds  to  the  maximum 
value  of  the  pressure  of  radiation  and  consequently 


256  The  Earth 

to  the  greatest  velocity  of  propulsion,  and  let 
this  particle  have  unit  density,  the  same  as  that 
of  water.  The  particle  will  be  subjected  to  the 
action  of  the  solar  gravity,  which  attracts  it 
towards  the  Sun,  and  to  that  of  the  repulsive 
radiation  pressure,  which  is  two  and  a  half  times 
greater  than  the  former.  A  mean  velocity  of  740 
kilometres  [450  miles]  per  second  is  thus  arrived 
at  by  calculation,  and  this  implies  that  the  particle 
takes  fifty-six  hours  to  pass  over  the  distance 
separating  the  Sun  from  the  Earth.  It  should  be 
remarked  that  we  have  assumed  the  density  of  the 
dust  particles  to  be  equal  to  that  of  water,  but  they 
have  in  all  probability  a  less  density  than  this, 
since  they  are  probably  formed  of  hydrocarbons 
containing  hydrogen  and  helium  in  solution.  If 
the  particles  considered  have  a  density  equal  to 
two  thirds  that  of  water,  the  calculation  gives 
the  result  that  under  the  resultant  repulsion  force 
the  particles  would  reach  the  Earth  from  the  Sun 
in  forty-five  hours. 

Now  Ricco  has  found  precisely  an  interval  of 
45  Yi  hours  between  the  passage  of  a  sun-spot  over 
the  solar  meridian  and  the  maximum  amplitude 
of  the  corresponding  magnetic  perturbation,  this 
result  being  based  on  half  a  score  of  clearly  observed 
cases;  in  another  series  he  arrived  at  an  interval 


Electric  and  Magnetic  PHenomena   257 

of  42^/2  hours.  This  constitutes  a  remarkable 
concordance  between  calculation  and  observa- 
tion, and  justifies  a  feeling  of  pride  that  Man  is 
able  so  to  overleap  the  apparent  limitations  of  his 
environment  and  attain  to  knowledge  from  which 
at  first  sight  he  would  seem  for  ever  debarred. 
The  explanation  of  the  mysterious  auroras  is  thus 
simple;  they  surround  the  pole  of  the  line  of  fall 
of  the  dust  particles  which  produce  them  and  so 
appear  to  shine  to  the  north  of  this  line  for  places 
which  are  exterior  to  it,  and  towards  the  south 
for  those  which  are  contained  within  it.  Physi- 
cists call  this  line  the  neutral  line. 

The  study  of  the  presence  of  the  solar  dust  in 
the  Earth's  atmosphere  enables  us  to  understand 
yet  another  thing.  The  negatively  charged  parti- 
cles driven,  as  we  have  already  seen,  from  the 
Sun  by  the  pressure  of  radiation,  meet  our  atmo- 
sphere and  discharge  themselves  to  earth,  pro- 
ducing aurorae.  This  discharge  of  negative 
electricity  communicates  to  the  Earth's  surface, 
and  maintains  an  electrostatic  negative  charge 
which  constitutes  what  is  known  as  atmospheric 
electricity.  A  study  of  this  phenomenon  has 
shown  that  the  electrical  potential  increases  in 
proportion  as  the  point  of  observation  is  higher 
above  the  ground;  in  the  neighbourhood  of  the 
n 


258  TKe  Earth 

ground  the  increase  of  potential  with  height  is, 
on  the  average,  150  volts  per  metre  [39.37  in.]. 
The  Earth's  charge  augments  when  the  solar 
spots  increase. 

We  cannot,  here,  enter  into  a  detailed  descrip- 
tion of  the  effects  of  atmospheric  electricity; 
lightning,  thunder,  electrification  by  induction 
and  the  effects  of  thunder-storms,  are  all  clearly 
described  in  works  on  elementary  physics  and  in 
popular  books.  What  we  have  to  remark  here  is 
that  there  appears  to  be  a  direct  bond  between 
solar  activity,  the  cause  of  the  Earth's  life,  and 
storms,  which  are  one  of  the  most  striking  mani- 
festations of  terrestrial  activity.  The  high  clouds 
which  float  in  the  atmosphere,  the  cirri,  are  formed 
in  great  abundance  as  a  consequence  of  the  pro- 
duction of  aurorae.  We  know  in  fact  that  when 
the  air  is  charged  with  water  vapour  and  when  also 
a  strong  ionisation  has  been  produced  under  the 
influence  of  the  cathode  rays,  condensation  is 
facilitated,  or,  in  other  words,  circumstances  are 
favourable  to  the  formation  of  clouds,  the  ions 
having  the  property  of  condensing  vapours. 
Abundance  o^  cirri  should  thus  accompany  the 
maxima  of  solar  spots.  Observations  during  fifty 
years  permit  us  to  state  that  there  is  an  agreement 
between  the  maxima  of  the  number  of  cirri  and 


Electric  and  Magnetic  Phenomena   259 

the  maxima  of  the  number  of  sun-spots,  the  periods 
of  both  phenomena  being  eleven  years. 

Cirri  may  also  be  electrified  by  the  action  of 
ultra-violet  rays.  These  rays  have  the  property 
of  rendering  gases  conductors  of  electricity,  that 
is  to  say,  of  ionising  them.  Furthermore  they 
discharge  the  negative  electrification  of  any  body 
they  fall  on,  while  not  affecting  the  positive  charge. 
Cirri,  which  are  formed  of  fine  needles  of  ice,  often 
pass  above  clouds  that  have  been  inductively 
electrified  by  the  proximity  to  the  ground;  in  this 
case  they  are  in  turn  subjected  to  the  inductive 
influence  of  these  clouds  and  their  component 
needles  are  charged,  negatively  at  one  extremity, 
positively  at  the  other.  In  these  conditions  if  a 
beam  of  cathode  rays  should  happen  to  strike  them 
their  negative  charge  would  be  dissipated,  and 
they  would  remain  positively  charged.  Here 
again,  consequently,  we  trace  to  the  Sun  an 
electrical  atmospheric  phenomenon. 

The  importance  of  these  electro-atmospheric 
phenomena  is  extremely  great,  especially  as  regards 
the  Earth's  animal  and  vegetable  life,  for  they 
determine  the  combination  of  the  nitrogen  of  the 
air  with  oxygen  and  hydrogen  and  accordingly 
give  rise  to  nitrates,  nitrites,  and  ammoniacal 
compounds,  the  great  importance  of  which  is  now 


26o  The  Earth 

understood  by  agriculturists.  These  compounds 
of  nitrogen  are  carried  down  to  the  soil  by  the 
action  of  rain,  and  in  this  way  more  than  400 
million  tons  are  brought  down  yearly. 

There  is,  finally,  another  electrical  property 
of  the  Earth,  viz. :  its  radioactivity.  At  the  com- 
mencement of  the  history  of  radioactive  pheno- 
mena, which  were  discovered  by  Becquerel,  in 
1896,  and  of  which  radium,  subsequently  dis- 
covered by  Mme.  Curie,  will  facilitate  the  study, 
it  was  believed  that  only  minerals  which  had  pro- 
duced radioactive  bodies,  for  example,  the  pitch- 
blende from  which  uranium  is  extracted,  possessed 
these  remarkable  properties.  We  now  know, 
however,  that  the  phenomenon  is  general,  and 
that  all  bodies  are  more  or  less  radioactive.  The 
terrestrial  crust  is  the  seat  of  a  radioactivity  which 
may  be  demonstrated  by  pushing  a  tube  into  the 
ground  for  a  depth  of  one  metre  [or  yard]  and 
breathing  the  air  found  there;  this  air  is  always 
more  or  less  charged  with  emanations.  It  follows 
that  the  air  in  caves  or  caverns  is  especially  so 
charged.  All  mineral  waters  are  radioactive,  as 
Professor  Moureu  has  discovered;  they  contain 
these  rare  gases  of  which  one,  helium,  has  been 
obtained  by  Sir  W.  Ramsay  as  a  product  of  the 
transformation  of  radium  emanation.  As  this 


Electric  and  Magnetic  Phenomena   261 

emanation  is  very  rapidly  dissipated,  this  explains 
why  the  greater  number  of  these  waters  are  only 
efficacious,  from  the  therapeutical  point  of  view, 
when  drunk  directly  from  the  source,  before  the 
gases  have  had  time  to  dissipate,  while  when  con- 
veyed to  a  distance  they  lose  all  the  gases  which 
constitute  the  chief  cause  of  their  curative  power 
and  become  merely  simple  saline  solutions.  Pro- 
fessor Moureu  has  even  shown  that,  excluding 
helium  which  is  one  of  the  products  of  radioactive 
emanations,  there  is  a  constant  mutual  ratio 
between  the  amounts  of  the  rare  gases  argon, 
krypton,  xenon,  neon  found  both  in  mineral 
springs  and  in  natural  gaseous  mixtures  such 
as  mine  emanations. 

This  is  readily  understandable;  at  the  origin 
of  the  Earth's  formation  by  the  process  of  con- 
densation, these  gases  did  not  combine  with  any 
other  of  the  elements  that  were  successively 
formed,  owing  to  the  chemical  inertia  of  the  former, 
inhibiting  the  production  of  compounds.  They 
therefore  remained  in  the  free  state  while  the 
other  elements  formed  combinations  among  them- 
selves; consequently  they  have  persisted  un- 
changed through,  and  unaffected  by,  all  the 
cataclysms  and  convulsions  which  have  marked 
the  successive  states  in  the  Earth's  history. 


262  The  Earth 

The  atmosphere  in  contact  with  radioactive 
soil  is  itself  radioactive.  Physicists  always  find 
there  traces,  of  course  infinitesimal,  of  the  emana- 
tion, and  freshly  fallen  rain  or  snow  invariably 
shows  signs  of  radioactivity;  it  is  the  same  with 
the  water  of  the  sea.  Concordant  experiments 
have  demonstrated  that  the  activity  of  a  gram 
[15.4  gr.]  of  radium  is  halved  in  about  2000  years; 
throughout  that  time  it  continues  to  emit  120 
calories1  per  hour,  say  in  round  numbers,  one 
million  calories  annually.  If,  therefore,  the  ter- 
restrial globe  contains  a  quantity  of  this  substance 
in  its  central  core  it  would  possess  a  considerable 
reserve  of  internal  heat.  How  can  radium  thus 
give  out  heat-energy  for  so  long  a  time?  Does  it 
absorb  some  kind  of  radiation  from  space  which 
it  is  able  to  transform  into  heat  by  unknown 
means? 

The  radiation  of  radioactive  bodies  comprises 
three  species  of  rays:  (i)  the  a-rays,  composed  of 
positively  charged  particles,  travel  approximately 
20,000  kilometres  [12,400  miles]  per  second;  these 
particles  are  atoms  of  helium;  (2)  the  p-rays, 
which  are  negative  electrons  whose  mass  is  nVo 
part  of  that  of  an  atom  of  hydrogen,  and  which 

TA  calorie  is  the  amount  of  heat  required  to  raise  I  gram 
15433  gr.]  of  water  i°  Centigrade  [  =  1.8°  F.].— Ed. 


Electric  and  Magnetic  Phenomena   263 

move  with  the  velocity  of  light1;  (3)  the  Y-rays, 
analogous  to  the  X-rays. 

Helium  is  always  found  in  radioactive  minerals 
and  it  is  derived  from  radium  emanation.  Helium, 
therefore,  seems  to  be  an  ultimate  element;  it 
occurs  as  the  final  product  of  the  disintegration 
of  other  atoms;  it  represents  in  fact  a  starting 
point  for  the  integration  of  the  more  complex 
atoms. 

Radioactive  substances — uranium,  actinium, 
radium — set  free  helium.  Sir  William  Ramsay 
has  proved  this  fact  in  regard  to  radium;  other 
experiments  have  enabled  him  to  ascertain  that 
copper  is  transformed  into  potassium,  sodium,  and 
lithium  and  also  that  lead,  thorium,  titanium,  and 
silicon  become  transmuted  into  carbon  under  the 
influence  of  the  energy  set  free  by  the  radium 
emanation.  If  these  experiments  be  confirmed, 
this  result  is  of  the  most  supreme  importance 
as  regards  the  theory  of  matter.  In  any  case, 
potassium,  sodium,  and  rubidium  are  feebly,  but 
distinctly,  radioactive.  It  thus  appears  that 
radioactivity  is  a  general  property  of  matter. 

Heavy  atoms  become  transformed  into  simpler 
ones,  losing  energy  in  the  process.  This  degrada- 

1  Velocity  of  light  =  300,000  kilometres  [186,000  miles]  per 
second.— Ed. 


264  The  Earth 

tion  is  spontaneous,  and  it  is  only  when  it  is  oc- 
curring with  a  slowness  that  renders  it  quite 
imperceptible  in  the  duration  of  our  existence  and 
experiments  that  we  consider  such  matter  as  stable. 

But  now  arises  the  question  as  to  the  origin  of 
the  heavy  atoms,  for  example  those  of  thorium 
or  uranium.  These  by  breaking  up  and  transfor- 
mation can  give  birth  to  those  of  lesser  atomic 
weight  but  cannot  themselves  arise  from  a  previous 
degradation.  It  is  therefore  natural  to  suppose 
that  they  arise  from  an  inverse  process  of  inte- 
gration of  matter,  starting  from  simple  atoms  such 
as  helium,  under  the  influence  of  considerable 
energy. 

We  are  here  led  back,  in  this  consideration  of 
the  origin  of  radioactivity  in  the  Earth's  crust, 
to  the  important  problem  of  the  age  of  our  globe. 
We  have  seen  in  a  preceding  chapter  that  the 
period  that  has  elapsed  since  the  formation  of  the 
solid  crust  lies  between  1000  and  2000  million 
years.  If  we  try  to  formulate  the  time  that  has 
passed  away  since  the  Earth  became  an  indepen- 
dent body,  after  its  detachment  from  the  nucleus 
of  the  solar  nebula,  we  must  reckon  at  least  a 
million  million  years. 

If  the  Earth  had  been  entirely  formed  of  uranium, 
a  million  million  years  would  have  been  a  more  than 


Electric  and  Magnetic  PHenomena   265 

sufficient  time  for  the  whole  of  it  to  be  transformed. 
Now  uranium  is  actually  found  in  the  Earth's 
crust,  whence  we  must  conclude  that  this  radio- 
active substance  is  formed  in  the  mass  of  our  globe. 
If  the  Earth  contained  5.000.000.000  of  a  gram 
of  radium  per  cubic  centimetre,  this  would  suffice 
to  prevent  its  cooling;  we  know  at  the  present 
time  sufficient  of  the  constants  of  radioactive 
material  to  make  such  a  calculation.  Now  ob- 
servation shows  that  the  radioactive  material  in 
the  terrestrial  crust  is,  on  the  average,  twenty 
times  greater  than  this.  Our  Earth  should  there- 
fore be  getting  hotter  and  the  deduced  duration 
of  the  geological  periods  would  be  increased  be- 
yond all  probable  limit.  Consequently  we  must 
assume,  as  the  English  scientists  have  done,  that 
the  whole  quantity  of  radioactive  material  present 
in  the  Earth  is  contained  in  a  very  thin  layer  of 
the  Earth's  crust  situated  in  the  immediate 
neighbourhood  of  its  exterior  surface.  The  thick- 
ness is  probably  only  a  very  few  kilometres  [or 
miles].  Thus  we  are  brought  to  a  difficulty.  Either 
the  interior  of  the  globe  contains  neither  uranium 
nor  thorium  or  else  the  heavy  atoms  of  these  sub- 
stances are  formed  there  by  the  integration  of 
matter  under  the  influence  of  the  colossal  pressures 
which  obtain  in  the  mass  of  the  central  nucleus. 


266  THe  EartK 

Moreover,  Arrhenius  has  considered  the  possi- 
bility of  formation,  in  the  central  parts  of  bodies 
whose  inner  regions  remain  heated,  of  endothermic 
compounds,  locking  up  an  immense  quantity  of 
energy,  truly  explosive  bodies  in  comparison  with 
which  dynamite  and  the  picrates  would  be  mere 
playthings ! 

In  discussing  the  origin  of  radioactive  substances 
we  have  thus  found  a  remarkable  consequence, 
viz.,  the  necessity  of  supposing  that  the  evolution 
of  matter  constitutes  a  cycle.  There  is  atomic 
decomposition  or  disintegration  on  the  one  hand, 
and  on  the  other  there  is  certainly  a  compensating 
integration,  which  assures  the  permanent  co- 
existence of  all  kinds  of  matter. 

All  the  facts  of  which  we  have  taken  note  in 
the  course  of  this  chapter  point  to  one  thing, 
viz.,  that  the  Earth  possesses  a  magnetic  state. 

What  is  this  state?  How  are  the  elements,  to 
which  the  magnetic  action  of  the  Earth  is  due, 
distributed  under  the  surface  on  which  we  live? 
How  are  they  arranged  in  such  a  way  as  to  show 
the  influences  of  solar  radiation?  These  are 
questions  to  which  the  science  of  Physics  can  at 
present  give  no  definite  answers. 

Fortunately,  however,  in  the  absence  of  answers 
which  could  be  furnished  by  some  great  theoretical 


Electric  and  Magnetic  PHenomena   267 

conception  that  has  yet  to  be  attained,  an  English 
scientist,  M.  H.  Wilde,  of  Manchester,  an  ingenious 
and  expert  experimenter,  to  whom  we  are  in- 
debted for  the  first  self -exciting  dynamo,  has 
constructed  a  wonderful  apparatus  which,  with  an 
almost  marvellous  exactness,  reproduces  not  only 
the  actual  distribution  of  magnetism  on  the  Earth's 
surface  but  even  the  secular  variations  of  this 
distribution  in  the  course  of  centuries.  This 
instrument  has  been  named  by  its  inventor,  the 
magnetarium.  Wilde  was  led  to  this  conception 
by  his  quite  original  cosmogonical  theory,  and  the 
accuracy  of  the  magnetic  results,  therefore,  also 
emphasises  the  value  of  this  theory.  Since,  for 
the  first  time,  all  the  peculiarities  of  such  a  complex 
phenomenon  as  that  of  the  distribution  of  terres- 
trial magnetism  and  its  secular  variations  have 
been  artificially  reproduced  in  the  laboratory,  the 
theoretical  ideas  which  led  to  such  an  achieve- 
ment cannot  be  valueless  and  merit  the  fullest 
attention  of  those  scientists,  who  by  the  aid  of 
mathematical  analyses  make  it  their  province 
to  construct  theories  concerning  the  origin  and 
functions  of  worlds. 

We  have  seen  in  studying  the  birth  of  the  Earth 
that  the  spheroidal  agglomeration  of  incandescent 
material,  which  at  a  later  period  constituted  our 


268  The  Earth 

planet,  gradually  cooled  in  such  a  way  as  to  be- 
come surrounded  by  a  superficial  solidified  layer. 
On  the  other  hand,  we  also  know  that  the  Earth 
while  traversing  its  orbit  remains  inclined  to  the 
plane  of  the  orbit,  the  angle  between  the  planes 
of  the  orbit  and  the  Earth's  equator  being  23^°. 
Wilde  holds  that  it  was  not  always  so  and  that 
at  a  certain  time,  extremely  long  ago  since  we  are 
considering  the  incandescent  phase  previous  to 
the  formation  of  the  solid  crust,  the  Earth  rotated 
about  an  axis  perpendicular  to  the  plane  of  the 
ecliptic.  In  these  circumstances  the  magnetic 
axis  of  the  system  of  electric  currents  arising  from 
the  solar  energy  would  be  parallel  to  the  polar 
axis  about  which  the  incandescent  spheroidal 
Earth  rotated.  At  a  later  period,  the  superficial 
solidification  occurred,  covering  the  mass  with  a 
rocky  crust.  Wilde  believes  that  at  that  time  the 
axis  of  rotation  about  which  the  crust  turned  was 
inclined  to  the  ecliptic  as  at  present,  while  the 
central  nucleus  continued  turning  about  the  axis 
of  its  original  rotatory  movement.  Thus,  accord- 
ing to  the  English  physicist,  in  place  of  a  single 
permanent  axis,  the  Earth  has  possessed  two  from 
the  time  of  the  formation  of  its  crust:  first,  the 
actual  axis,  serving  as  axis  only  for  the  solid  en- 
velope; secondly,  the  primitive  axis  upon  which 


Electric  and  Magnetic  Phenomena    269 

turns  the  igneous  mass  that  constitutes  the  central 
nucleus  of  the  globe.  Furthermore,  Wilde  has 
arrived  at  the  conclusion  that  this  internal  mass 
rotates  about  the  primitive  axis  with  a  smaller 
angular  velocity  than  that  of  the  crust  turning 
about  the  inclined  axis. 

Another  point  of  the  theory  is  that  the  super- 
ficial layers  became  magnetic  as  they  cooled,  the 
magnetisation  taken  as  a  whole  being  parallel  to 
the  inclined  axis,  so  long  as,  at  the  time  of  its 
solidification,  the  exterior  surface  of  the  crust 
remained  almost  level.  But  from  the  time  when 
the  crust  became  subject  to  foldings  and  con- 
tractions, its  resulting  deformations  produced  a 
magnetisation  of  very  great  complexity. 

To  summarise,  Wilde  holds  that  the  terrestrial 
magnetism  is  the  resultant  of  two  component 
elements,  one  connected  with  the  actual  constitu- 
tion of  the  Earth's  solid  envelope,  the  other  due 
to  interior  currents  having  as  axis  of  symmetry  a 
line  inclined  to  the  axis  of  rotation  of  the  crust, 
slowly  describing  a  cone  around  the  latter,  on 
account  of  the  inequality  of  the  velocities  of 
rotation  of  nucleus  and  crust. 

In  order  to  arrive  at  a  material  representation 
of  this  complex  phenomenon  the  English  physicist 
took  a  sphere  like  those  which  form  terrestrial 


270  The  EartK 

globes.  This  globe  was  mounted,  as  are  the  greater 
number  of  those  used  for  teaching  purposes,  in 
such  a  way  that  it  rotated  about  an  axis,  the  two 
extremities  of  which  were  supported  by  a  copper 
semicircular  arc,  forming  a  semi-meridian.  This 
arc  is  itself  capable  of  sliding  in  its  own  support 
in  such  a  way  that  any  point  whatever  of  the 
Earth's  surface  may  be  brought  to  have  the  same 
horizon  and  the  same  zenith  as  the  place  of  the 
experiment.  A  rigid  arm,  fixed  to  the  support  of 
the  entire  apparatus,  enables  either  a  small  in- 
clination needle,  or  a  small  declination  needle  to 
be  placed  above  the  point  so  chosen ;  consequently 
the  elements  of  the  artificial  magnetism  given  to 
this  magnetic  globe  may  be  experimentally 
measured. 

In  order  to  give  his  globe  magnetic  properties 
Wilde  supplied  it  with  a  series  of  insulated  wires 
wound  according  to  the  parallels  of  latitude  in 
such  a  way  as  to  constitute  a  sort  of  spherical 
bobbin.  It  follows  from  the  laws  of  electro- 
magnetism  that  in  this  case  the  system  behaves 
like  a  sphere  magnetised  in  a  direction  parallel  to 
its  axis  of  rotation. 

This  first  globe  contains  a  second  one  turning 
about  a  hollow  axis  enclosing  the  axis  of  the  outer 
globe.  The  inner  globe  is  also  covered  with  wire 


Electric  and  Magnetic  PHenomena   271 

so  as  to  resemble  a  spherical  bobbin,  but  it  is  not 
wound  according  to  the  parallels  of  latitude;  it 
is  so  wound  that  the  poles  of  the  spirals  are  the 
two  extremities  of  a  diameter  making  an  angle  of 
1 8°  with  the  axis  of  rotation,  viz.,  the  difference 
in  latitude  between  the  North  Geographical  and 
North  Magnetic  Poles  of  the  Earth.  A  mechanism 
with  a  differential  train  of  wheels  enables  the  two 
globes  to  be  made  to  turn  simultaneously  in  such 
a  way  that  the  interior  globe  is  subjected  to  an 
angular  retardation  of  12°  in  each  turn  relatively 
to  the  outer  globe.  In  these  circumstances  it 
may  be  shown  that  the  system  is  equivalent  to  a 
magnet,  the  line  of  whose  poles  is  inclined  to  the 
Earth's  axis  at  an  angle  less  than  18°  and  which 
rotates  continuously  about  that  axis,  the  two 
bobbins  being  traversed  by  suitable  currents 
supplied  to  them.  Determinations  of  the  declina- 
tion and  inclination  for  different  places  on  the 
globe's  surface  were  made  with  the  above  arrange- 
ment by  the  little  test  needles,  but  the  result  did 
not  come  up  to  expectation. 

The  idea  then  occurred  to  Wilde  of  altering  the 
inclination  of  the  magnetic  axis  of  the  interior 
globe  so  as  to  make  an  angle  of  23^°,  instead  of 
1 8°,  with  that  of  the  exterior  globe.  In  other 
words  the  two  axes  made  an  angle  with  each  other 


272  The  Earth 

equal  to  that  between  the  planes  of  the  terrestrial 
equator  and  the  ecliptic;  consequently  at  certain 
periods  of  the  movement  the  magnetic  axis  of  the 
interior  globe  became  perpendicular  to  the  plane 
of  the  terrestrial  orbit.  The  instrument  so 
arranged  showed  approximately  the  successive 
values  of  the  magnetic  elements  observed  at 
London.  One  complete  turn  of  the  exterior  globe 
corresponded  to  an  angular  displacement  of  12° 
with  reference  to  the  interior  globe,  which  corre- 
sponds to  an  interval  of  time  of  thirty-two  years. 
We,  therefore,  draw  the  conclusion  that  the  general 
period  of  the  secular  variation  is  about  960  years. 
The  results,  however,  although  giving  fairly 
exact  values  for  the  magnetic  elements  at  London 
showed  notable  discrepancies  for  other  places  on 
the  Earth  when  compared  with  the  actual  values 
observed  at  these  places.  Also,  the  distribution 
of  magnetic  meridians  and  isogonal  lines  on  the 
magnetarium  was  more  regular  than  the  actual 
terrestrial  ones.  In  order  to  remedy  this  Wilde 
conceived  the  very  original  idea  of  covering  those 
portions  of  the  surface  of  the  magnetarium  which 
represented  the  oceans  with  layers  of  sheet  iron  of 
a  suitable  thickness  cut  to  shape.  The  result  was 
remarkable.  Not  only  did  the  instrument  re- 
produce exactly  the  actual  values  of  the  magnetic 


Electric  and  Magnetic  PHenomena   273 

elements  at  the  various  portions  of  the  Earth's  sur- 
face, even  for  stations  as  widely  separated  from  one 
another  as  London,  the  Cape,  and  St.  Helena,  but 
it  also  showed,  for  the  same  stations,  the  secular 
variations  of  the  elements.  Furthermore  it  even 
reproduced  the  oval  of  Eastern  Siberia,  in  the 
interior  of  which  the  declination  is  westward, 
and  also  the  oval  of  minimum  declination  observed 
in  the  east  of  the  Pacific  near  the  neighbourhood 
of  the  equator. 

Thus,  for  the  first  time,  a  natural  phenomenon 
of  so  great  a  complexity  as  that  of  terrestrial 
magnetism,  has  been  reproduced  artificially  in 
every  detail,  not  only  as  regards  its  distribution 
in  space  but  also  showing  the  secular  variations 
which  occur  in  time.  We  cannot  look  with  in- 
difference upon  the  theoretical  considerations 
which  have  led  to  a  result  that  is  so  remarkably 
in  accordance  with  the  natural  phenomenon.  In 
particular,  what  is  the  reason  of  the  role  of 
magnetic  screen  played  by  the  seas?  We  know 
that  the  oceans  exert  a  very  great  influence  upon 
the  atmospheric  circulation  and  climatology  in 
general.  What  is  the  nature  of  their  mysterious 
influence  upon  the  distribution  of  terrestrial 
magnetism?  Perhaps  it  is  a  consequence  of  a 
state  of  affairs  corresponding  to  Lippmann's 

18 


274  The  Earth 

theory,  viz.,  that  the  thickness  of  the  terrestrial 
crust  is  less  under  the  oceans,  so  that  in  these 
parts  the  internal  ferruginous  materials  are  nearer 
the  surface  of  the  geoid  than  elsewhere,  and 
consequently  play  the  same  part  as  Wilde's  screens. 

So  once  again,  as  we  have  already  seen  in  the 
study  of  gravity  and  the  form  of  the  Earth,  and 
as  we  shall  see  later  in  connection  with  general 
meteorology,  the  solution  of  the  problems  of  the 
physics  of  the  globe  will  probably  be  found  in  the 
study  of  the  oceans.  Prince  Albert  of  Monaco 
clearly  foresaw  this  when  he  founded  the  Oceano- 
graphical  Institute. 

In  any  case,  we  see  the  importance  of  the  initial 
conception  of  distinguishing  the  magnetic  action 
of  the  nucleus  from  that  of  the  crust.  Bauer, 
who  superintends  the  magnetic  work  of  the 
Carnegie  Institution,  also  believes,  as  a  result  of 
studying  Gauss's  conclusions,  that  the  terrestrial 
magnetism  resides  almost  entirely  in  the  solid 
crust  which  envelops  the  nucleus. 

To  summarise,  the  Sun's  action  is  paramount  as 
regards  the  electric  and  magnetic  phenomena  of 
which  the  terrestrial  globe  is  the  seat.  We  do  not 
yet  know,  however,  whether  these  phenomena  are 
wholly  due  to  the  solar  field  or  whether  the  latter 
merely  modifies  them.  In  other  words,  is  the 


Electric  and  Magnetic  PHenomena   275 

Sun's  action  sufficient  to  give  rise  to  the  Earth's 
magnetic  field  or,  on  the  other  hand,  does  this 
latter  pre-exist,  caused  by  some  original  and  as 
yet  unknown  cause,  only  its  variations  arising 
from  the  variations  of  the  solar  radiation?  What 
is  quite  certain,  is  the  actually  proved  connection 
between  the  variation  in  the  number  of  solar  spots 
on  the  one  hand  and  magnetic  variations,  varia- 
tions of  earth  currents,  of  magnetic  storms,  of  the 
number  of  polar  aurora,  and  frequently  of  the 
number  of  seismic  phenomena.  This  shows  that, 
even  if  we  must  not  seek  the  cause  of  terrestrial 
magnetism  in  the  Sun  (which  we  have  not  yet 
been  able  to  prove),  we  must,  at  any  rate,  look 
there  for  the  cause  of  the  variations  to  which  its 
numerous  phenomena  are  subjected. 


CHAPTER  IX 

THE  RHYTHMIC  MOVEMENTS  OF  THE  OCEAN,  TIDES, 
SWELL,  AND  WAVES 

'"THE  manifestations  of  the  life  of  the  terrestrial 
*  globe,  its  general  movements,  the  perturba- 
tions it  is  subjected  to,  the  spasms  of  its  crust, 
and  the  manifestation  of  electricity  and  magnetism 
which  traverse  it,  as  well  as  all  that  communicates 
this  incessant  restlessness  to  the  Earth,  have  their 
origin  in  the  Sun. 

But  our  globe  is  not  composed  entirely  of  its 
lithosphere;  the  hydrosphere  which  covers  more 
than  three-fourths  of  its  surface  has  an  importance 
of  which  we  have  already  seen  something,  for 
example,  in  connection  with  the  distribution  of 
terrestrial  magnetism.  We  shall  now  study  the 
movements  of  the  hydrosphere,  and  here  again  we 
shall  find  evidence  of  solar  influence,  either  the 
direct  action  of  the  attraction  of  its  mass,  or  the 
indirect  action  of  the  heating  of  the  molecules  of 
the  fluid  substances  water  and  air,  enveloping  the 
Earth's  crust,  and  from  which  results  the  general 

276 


RHytHmic  Movements  of  tHe  Ocean     277 

movements  that  are  the  origin  of  the  circu- 
lation of  the  oceans  and  the  circulation  of  the 
atmosphere. 

It  is  hardly  necessary  to  recall  the  facts  concern- 
ing the  importance  of  the  sea  in  the  general 
economy  of  the  globe.  In  the  first  place  it  occupies 
more  than  two-thirds  of  the  surface;  out  of  the 
510  millions  of  square  kilometres  [194  millions 
sq.  miles]  of  the  terrestrial  crust  365  millions 
[138  millions  sq.  miles]  are  covered  by  water. 
There  is  thus  far  more  water  than  land,  and  to  the 
oceans,  like  bodies  elected  by  universal  suffrage, 
must  accrue  the  rights  of  the  majority.  The 
great  atmospheric  conditions  become  established, 
not  above  the  smaller  part  of  the  Earth's  surface 
exhibiting  the  innumerable  accidents  of  the 
geographical  relief,  but  above  the  vast  uniform 
oceanic  surface  the  molecules  of  which  freely  obey 
the  laws  of  fluid  mechanics. 

The  total  volume  of  the  water  of  the  oceans  is 
about  1300  million  cubic  kilometres  [309  million 
cubic  miles],  while  that  of  the  emergent  dry  land  is 
only  100  million  [24  million  cubic  miles].  The 
mean  depth  of  the  seas  is  about  3550  metres 
[2.2  miles].  All  this  mass  of  water  contains  a 
quantity  of  salts  in  solution  and  also,  doubtless, 
metals  in  a  state  of  extremely  fine  division.  This 


278  TKe  EartH 

will  be  readily  understood  since,  the  seas,  in  the 
first  instance,  were  formed  by  the  collection  of 
the  boiling  water-streams  which  condensed  from 
the  Earth's  primitive  atmosphere  and  were  pre- 
cipitated on  the  scarcely  solidified  crust.  In  such 
conditions,  the  water  would  have  dissolved  all 
that  was  soluble  on  the  surface  of  the  Earth. 
Sea-water  should  thus  contain  all  known  sub- 
stances, at  any  rate  in  traces.  The  mean  quantity 
of  salts  contained  in  a  kilogram  [2  Ib.  3  oz.  4  dr.] 
of  sea-water  is  35  grams  [1.25  oz.]  and  75%  of  this 
total  salinity  is  composed  of  sodium  chloride,  that 
is  to  say,  common  salt.  The  salt  in  solution  in  all 
the  seas  would  provide  enough  material  to  con- 
struct the  African  continent  in  all  its  relief.  With 
the  gold,  of  which  only  a  few  milligrams  [a  few 
hundredths  of  a  grain]  are  contained  in  a  ton  of 
water,  a  block  could  be  made  which,  if  divided 
equally  among  every  inhabitant  of  the  Earth, 
1,500,000,000  in  number,  would  give  to  each  one 
an  ingot  of  40,000  kilograms  [44  tons]  of  the  precious 
metal,  or  in  other  words  a  fortune  of  120  million 
francs  [24  million  dollars]! 

The  salinity  of  the  oceans  increases  their  den- 
sity; a  litre  [1.05  quarts]  of  sea- water  weighs  I 
kilogram  [2  Ib.  3  oz.  4  dr.]  and  28  grams  [432.1  gr.] 
instead  of  I  kilogram  as  pure  water  does.  One 


RHytHmic  Movements  of  tKe  Ocean     279 

can  therefore  swim  more  easily  in  it  as  it  possesses 
a  greater  buoyancy. 

The  greatest  depth  revealed  in  the  course  of  the 
Pacific  soundings  is  actually  9750  metres  [6  miles]. 
The  continents  seem  to  lie  on  a  kind  of  base, 
known  as  the  continental  plateau,  the  mean  dis- 
tance of  which  below  the  surface  is  200  metres 
[650  ft.].  Beyond  the  immediate  neighbourhood 
of  the  continents,  the  depth  rapidly  increases,  and 
this  applies  equally  to  the  Atlantic,  the  Pacific,  and 
the  Indian  oceans.  In  the  case  of  the  Mediter- 
ranean, the  Straits  of  Gibraltar,  dominated  by  the 
Rock,  impose  special  temperature  conditions,  but 
in  all  other  cases  the  temperature  falls  in  propor- 
tion to  the  depth  below  the  surface,  and  when 
depths  of  6000,  7000  and  8000  metres  [3.75, 
4.35,  5  miles]  are  reached  it  is  found  that  the 
water  there  has  a  uniform  temperature  in  the 
neighbourhood  of  zero  [o°C.  =  32°  FJ.  Here  we  find 
ourselves  in  the  presence  of  one  of  the  paradoxes 
of  terrestrial  physics.  If  we  bored  into  the 
Earth's  crust  to  a  depth  of  8000  metres  [5  miles] 
we  should  find  a  temperature  of  about  240°- 
250°  C.  [400°-420°  P.]  at  the  bottom  of  the  shaft 
so  made,  while  at  the  same  depth  under  the  seas 
the  result  is  o°  C.  [32°  F.]!  The  difficulty  is  in- 
creased by  the  fact  that  the  crust,  which  is 


280  The  Earth 

presumably  of  less  thickness  under  the  oceans, 
consequently  offers  less  resistance  to  the  trans- 
mission of  the  internal  heat.  The  explanation  is 
doubtless  to  be  sought  for  in  the  stream  of  cold 
water  coming  from  the  polar  regions,  which  on 
account  of  its  greater  density  falls  to  the  bottom 
of  the  great  oceanic  hollows.  Then  when  the 
water  becomes  warmed  to  above  zero  [32°  P.],  it 
rises  by  a  process  of  convection,  thus  producing 
a  vertical  oceanic  circulation;  the  water  so  coming 
up  is  replaced  by  more  cold  water  from  the  polar 
regions. 

The  movements  of  these  waters  constitute  one 
of  the  most  imposing  manifestations  of  the  "life" 
of  the  globe ;  by  passing  a  few  days  on  the  Brittany 
coast  we  can  not  only  admire  the  magnificent 
phenomena  of  the  tides,  but  are  also  enabled  to 
conceive  the  immediately  apparent  laws  governing 
them. 

We  see,  at  a  certain  time,  the  level  of  the  sea  rise 
in  a  continuous  manner,  constituting  what  is 
called  the  rising  tide ;  at  the  same  time  the  water 
of  the  open  sea  advances  towards  the  land,  and 
this  current  is  known  as  the  flood-tide  or  the  flow. 
Gradually,  the  rise  of  level  stops  and  the  flow 
ceases,  the  water  finally  remains  stationary  at  its 
highest  point.  It  is  now  the  time  of  high  tide. 


RHytHmic  Movements  of  tHe  Ocean     281 

Subsequently  the  current  recommences  in  the 
opposite  direction,  the  water  flowing  from  the  land 
towards  the  sea;  this  is  the  ebb.  The  level  of  the 
sea  gradually  falls,  and  that  part  of  the  land 
covered  by  the  rising  tide  reappears.  The  falling 
tide  becomes  more  rapid  up  to  a  certain  limit, 
and  then  its  rate  decreases  and  finally  ceases 
altogether,  the  water  having  reached  its  lowest 
level.  This  is  the  epoch  of  low  tide.  Soon  the 
process  of  rise  begins  again  and  passes  through  all 
its  former  phases,  constituting  a  second  high  tide, 
followed  by  a  second  low  tide  some  hours  after. 

If  the  phenomenon  be  watched  for  several  days 
it  maybe  shown  that  there  are,  broadly  speaking, 
two  high  tides  and  two  low  tides  daily,  but  the 
interval  of  time  between  them  is  not  an  exact 
subdivision  of  a  day.  Thus,  if  a  high  tide  be 
noted  at  eight  o'clock  on  the  morning  of  a  certain 
day,  the  high  tide  of  the  next  day  will  not  occur 
at  eight  o'clock  but  at  ten  minutes  to  nine.  The 
interval  separating  the  two  high  tides  which  takes 
place  during  the  same  day  is  not  12  hours  but 
12  hours  and  25  minutes,  the  difference  being 
half  the  above.  The  diurnal  period  of  the  tide 
is  thus  24  hours  50  minutes.  Now,  this  is 
precisely  the  value  of  the  interval  of  time  separ- 
ating two  successive  passages  of  the  Moon  over 


282  TKe  Earth 

the  meridian  of  the  place.  We  hence  see  that  the 
Moon  is  a  dominating  factor  in  tide-production. 

It  will  further  be  observed  that  at  any  one 
place  on  the  coast  the  water  at  the  moment  of 
highest  tide  is  never  at  quite  the  same  level  on 
any  two  following  days.  For  several  days  the 
tides  increase  in  height  from  one  day  to  another; 
this  is  the  period  of  spring- tides.  Then  follows  a 
period  when  the  level  of  the  high  tides  is  less  each 
day;  this  is  the  time  of  neap-tides.  Observation 
shows  that  the  periodicity  of  spring-tides  and  neap- 
tides  is  the  same  as  that  of  the  phases  of  the  Moon, 
and  these  latter  depend  on  the  relative  positions 
of  the  Moon  and  the  Sun  with  regard  to  the  Earth. 
Consequently,  although  the  Moon  is  the  principal 
factor  which  governs  the  tide  period,  the  Sun 
also  has  an  effect  which  modifies  the  magnitude 
of  the  phenomenon. 

There  is  another  observed  fact  which  is  note- 
worthy, viz.,  that  the  height  of  the  tide  may  have 
very  different  values  on  the  same  day  at  two 
points  of  the  Earth  which  are  close  to  one  another 
and  which  are  therefore  at  the  same  distance  from 
the  attracting  bodies.  For  example,  if  we  find 
a  tide  at  Granville,  of  6.11  metres  [20  ft.]  on  a 
certain  day,  the  height  of  the  tide  at  the  neigh- 
bouring port  of  Cherbourg  on  the  same  day  will 


RHytHmic  Movements  of  tHe  Ocean     283 

only  be  2.82  metres  [9.2  ft.].  There  is,  therefore,  a 
geographical  factor  which  influences  the  pheno- 
menon and  which  arises  from  the  coastal  con- 
figuration at  the  place  considered.  Finally,  it 
may  be  proved  that  high  tide  at  any  given  place 
does  not  take  place  exactly  according  to  the  astro- 
nomical attractions  arising  from  the  position  of  the 
Moon  and  of  the  Sun;  it  takes  place  some  time 
afterwards  and  the  time-interval  of  retardation  is 
constant  for  each  place.  For  any  port  this  is 
called  the  establishment  of  the  port.  The  great- 
est establishment  of  the  port  in  France  is  12 
hours  30  minutes,  at  Dunkerque;  the  smallest  is  at 
Lorient,  viz.,  3  hours  32  minutes.  Here  again  the 
geographical  configuration  of  the  coasts  and  the 
irregularity  of  the  sea-bottom  have  an  important 
influence. 

It  was  Newton  who  first  gave  the  explanation  of 
the  beautiful  phenomena  of  the  tides.  The  Moon, 
on  account  of  its  proximity  to  the  Earth,  attracts 
the  molecules  of  the  water  in  the  oceans  that  are 
situated  on  the  side  of  the  Earth  facing  it,  to  a 
greater  extent  than  it  attracts  the  centre  of  the 
globe  and  this  latter  is  attracted  more  than  the 
molecules  of  water  situated  on  the  opposite  side 
of  the  Earth.  We,  thus,  find  at  the  free  surfaces  of 
the  seas  two  liquid  protuberances,  the  summits 


284  The  EartH 

of  which  are  situated  on  the  line  which  joins  the 
centre  of  the  Moon  to  the  centre  of  the  Earth. 
The  first  is  due  to  the  attraction  towards  the 
Moon  of  the  fluid  mass  lying  on  the  side  nearest 
it ;  the  second,  on  the  opposite  side  arises  from  the 
fact  that  the  centre  of  the  globe  is  more  strongly 
attracted,  being  nearer  the  Moon,  than  the  water 
on  the  far  side  which  is,  so  to  speak,  left  behind 
and  hence  forms  a  protuberance  (Fig.  27). 


FIG.  27. — Tidal  Prominences  produced  upon  the  Seas  by  the 
Attraction  of  a  Neighbouring  Body. 

When  the  Sun  is  in  the  same  direction  as  the 
Moon  with  respect  to  the  Earth,  that  is  to  say  at 
the  epoch  of  syzygy,  their  attractive  forces  are 
additive;  when  the  two  bodies  have  their  centres 
on  the  two  sides  of  a  right  angle  formed  at  the 
centre  of  the  Earth,  that  is  to  say  at  the  epoch  of 
quadrature,  the  attractions  oppose  one  another. 
We  have  seen,  in  studying  the  deviation  from 
the  vertical  under  the  influence  of  the  luni -solar 
attraction,  that  if  we  represent  the  Sun's  attrac- 


RHytHmic  Movements  of  tKe  Ocean     285 

tion  by  I,  that  of  the  Moon  is  approximately 
equal  to  2.  The  tide  would  thus  have  theoreti- 
cally for  relative  amplitudes  2  +  I,  i.  e.,  3  at  the 
epochs  of  spring-tides  and  2  —  1,  i.e.,  I  at  the  epochs 
of  neap-tides. 

This  explanation  of  Newton,  based  on  the 
equilibrium  between  the  lunar  attraction  and 
gravity,  gives  an  account  of  the  phenomenon  in 
its  broad  outlines  and  general  details,  but  is  found 
wanting  when  certain  observational  facts  are 
taken  into  consideration.  For  example,  if  we 
apply  the  theory  of  static  equilibrium  to  the 
calculation  of  the  height  of  the  liquid  protuberance 
which  represents  the  tide,  we  obtain  a  result  of 
35  centimetres  [13.75  in.].  Now,  the  most  cur- 
sory observation  shows  that  the  variations  of  the 
level  of  the  sea  under  tidal  influence  have  much 
greater  values  than  this.  In  the  ports  of  the 
English  Channel  and  Brittany  the  variation  is 
several  metres;  at  Mont  Saint-Michel,  at  the  epoch 
of  spring- tides,  the  difference  reaches  14  metres 
[46  ft.],  while  in  the  Straits  of  Magellan  it  attains 
1 8  metres  [59  ft.],  and  in  the  Bay  of  Fundy,  on 
the  coast  of  Newfoundland,  21  metres  [69  ft.].1 


1  At  Chepstow  on  the  River  Wye  in  England  occurs  the  high- 
est tide  in  the  British  Isles  and  probably  in  Europe;  it  has  been 
known  to  attain  a  height  of  forty- seven  ft. — Trans. 


286  TKe  Earth 

Furthermore,  in  certain  regions,  for  example,  in 
Polynesia  or  in  the  Gulf  of  Tonkin,  there  is  only 
one  tide  daily  instead  of  two. 

Consequently  the  simple  consideration  of  equi- 
librium between  the  astronomical  attractive 
forces  and  the  terrestrial  gravity  do  not  suffice 
to  explain  the  amplitude  of  the  tide;  neither  do 
they  satisfactorily  account  for  the  observed 
retardations,  nor  for  the  differences  between 
two  neighbouring  places.  It  will,  therefore,  be 
necessary  to  seek  for  the  complete  explanation  of 
the  tides,  not  in  the  law  of  fluid  equilibrium,  but 
in  that  of  their  movements,  or  in  other  words 
not  in  the  study  of  hydrostatics,  but  in  that  of 
hydrodynamics. 

We  shall  once  more  find  Laplace's  genius  the 
starting  point  of  this  theory.  The  illustrious 
mathematician  recognised  that  when  the  luni- 
solar  attraction  influences  the  water  of  the  oceans, 
this  attraction,  which  is  exercised  by  two  bodies 
that  move  with  respect  to  the  Earth,  should  give 
rise  not  to  a  fixed  protuberance,  a  liquid  hill  so  to 
speak,  but  to  a  true  undulation  which  would  move 
over  the  sea  surface  in  accordance  with  some  more 
or  less  complex  law,  depending  on  the  nature  of  the 
relative  movements  of  the  attracting  bodies,  on  the 
angular  variations  of  their  positions  with  regard 


RHytHmic  Movements  of  tKe  Ocean    287 

to  the  Earth,  and  on  the  variations  of  actual 
distance  from  the  Earth.  Thus,  since  we  are  led 
to  consider  wave  propagation,  we  must  study  the 
matter  from  the  point  of  view  of  fluid  dynamics, 
taking  account  of  the  resistances  of  fluids.  Two 
fundamental  principles  govern  this  study,  first 
that  of  the  superposition  of  small  movements 
and  secondly,  that  of  periodicity.  The  enunciation 
of  the  first  is  as  follows:  Let  us  start  with  the 
assumption  that  a  system  of  material  points  is 
in  equilibrium,  and  that  a  very  small  force  is 
applied  so  as  to  disturb  this  equilibrium.  Then, 
a  material  point  will  be  given  a  small  velocity,  so 
small  that  the  expression  of  the  force  depends  only 
on  the  time  and  the  mean  position  of  the  point. 
In  these  conditions,  if  several  forces  act  simul- 
taneously, the  laws  of  mechanics  show  that  at 
each  instant  their  effects  are  independent  and 
consequently  superimposable.  Also,  as  these 
momentary  effects  have  no  influence  upon  the 
forces  themselves,  it  follows  that  the  total  effect 
will  be  the  sum  of  the  partial  efforts  calculated 
as  if  each  force  acted  separately. 

This  first  principle  is  of  extreme  importance ;  it 
enables  us  to  consider  separately  the  influence  of 
the  Moon  and  the  Sun.  That  is  if  we  evaluate 
on  the  one  hand  the  solar  tide  and  on  the  other 


288  The  EartH 

hand  the  lunar  tide,  we  get  the  resultant  tide  by 
adding  the  two  together.  All  optical  and  acoustic 
phenomena  are  illustrations  of  this  principle; 
light  and  sound  waves  proceed  through  space 
without  mutual  interference,  and,  at  the  present 
time,  trains  of  electric  waves  rapidly  travel  above 
the  Earth's  surface,  propagating  themselves  in  all 
directions  without  the  one  in  any  way  hindering 
the  others. 

The  second  principle  is  that  of  the  periodicity 
of  movements  caused  by  periodic  forces:  Every 
periodic  force  produces  periodic  movements  in 
the  group  of  molecules  on  which  it  acts.  The 
periods  of  the  force  and  resulting  movement  are 
equal  and  at  a  given  point  their  difference  of 
phase  is  constant.  Thus,  a  body  which  travels 
uniformly  in  the  plane  of  the  equator,  remaining 
always  at  an  invariable  distance  from  the  Earth, 
would  give  rise,  by  its  relative  diurnal  movement, 
to  perturbing  forces  having  a  period  of  half  a  day. 
In  any  place  whatever,  the  variations  of  the  level 
of  the  sea  resulting  from  this  action  would  have, 
according  to  the  second  principle  above,  exactly 
the  same  semi-diurnal  period,  and  the  diverse 
phases  of  the  movements  would  be  displaced 
relatively  to  the  corresponding  phases  of  the 
force  by  a  constant  interval  of  time. 


RHytHmic  Movements  of  tKe  Ocean    289 

The  relative  movement  of  the  attracting  bodies, 
however,  does  not  take  place  in  so  simple  a  manner 
as  this,  and,  leaving  the  other  elements  out  of 
consideration,  the  fact  of  the  inclination  of  their 
apparent  paths  to  the  plane  of  the  terrestrial 
equator  necessitates  the  addition  of  two  other 
categories  of  forces  to  those  of  semi-diurnal 
period.  These  other  forces  are  diurnal  and  long 
period  forces  respectively. 

In  the  first  place,  the  action  of  a  body  turning 
only  around  the  Earth  will  be  of  semi-diurnal 
period,  for  the  result  is  the  production  of  two  tide 
wave-summits  diametrically  opposite  one  another. 
The  terrestrial  diameter  which  joins  these  two 
summits  follows  the  orbital  movement  of  the  body. 
In  these  circumstances,  the  two  waves  travel 
around  the  Earth  in  such  a  way  that,  in  the  course 
of  the  twenty-four  hours,  which  is  by  supposition 
the  period  of  the  body's  movement,  a  point  A  of 
the  Earth  will  be  twice  affected  by  the  tide  wave, 
viz.,  once  by  the  wave  M  and  once  by  the  wave  M' 
(Fig.  27).  The  period  will  be  semi-diurnal.  This 
is  what  would  occur  if  the  body  was  in  the  plane 
of  the  equator  and  at  a  constant  distance  from  the 
Earth. 

But  the  body  is  not  usually  in  the  plane  of  the 
terrestrial  equator.  In  the  general  case  it  is 


290  TKe  EartH 

outside  this  plane,  either  above  or  below  it. 
Consequently  the  two  zones  of  deformation  M  and 
M',  through  which  every  point  of  the  Earth  passes 
in  its  diurnal  movement,  are  unsymmetrical. 
These  two  protuberances  have  therefore  unequal 
effects,  and  this  inequality  is  represented  analyti- 
cally by  the  superimposition  of  a  movement  of  the 
sea,  of  diurnal  period,  on  the  semi-diurnal  wave  of 
which  we  have  previously  spoken.  We  already 
see  that  there  will  be  a  considerable  number  of 
waves;  in  fact,  if  we  take  as  unity  the  solar  day, 
we  shall  have,  on  the  one  hand,  to  express  the  Sun's 
action,  a  solar  day  of  24  hours  and  a  semi-solar 
day,  and  for  the  Moon,  on  the  other  hand,  a  lunar 
day  of  24  hours  50  minutes  and  a  semi-lunar  day 
of  12  hours  25  minutes. 

The  orbits  of  the  two  bodies  are,  furthermore, 
inclined  to  the  plane  of  the  terrestrial  equator; 
the  charges  to  which  the  declinations  of  the  Moon 
and  Sun  are  subjected  from  one  day  to  the  next 
are  accompanied  by  little  variations  in  the  respec- 
tive durations  of  the  true  solar  day  and  the  lunar 
day.  This  has  an  effect  on  the  amplitude  and  period 
of  the  diurnal  and  semi-diurnal  undulations.  We 
must  next  note  that  the  Moon  and  the  Sun  are 
not  at  constant  distances  from  the  Earth,  owing 
to  the  ellipticity  of  the  orbits  of  the  Moon  and 


RHytHmic  Movements  of  the  Ocean     291 

Earth.  There  is  a  variation  to  the  extent  of 
^s  part  in  the  case  of  the  Moon  in  a  month, 
and  one  of  6\  part  in  the  distance  of  the  Sun 
from  the  Earth  in  a  year.  This  is  another  cause 
of  modification  of  the  amplitude  of  the  solar  and 
lunar  undulations,  and  the  result  is  as  if  we 
combined  with  each  of  the  principal  waves, 
of  their  average  amplitude,  two  subsidiary  waves 
having  as  amplitude  the  semi-difference  between 
the  mean  and  the  maximum,  and  for  respective 
velocities  the  sum  of  and  the  difference  between 
the  principal  wave  and  the  variation  of  amplitude. 

We  have  already  mentioned  that  long  period 
waves  are  superimposed  upon  these  diurnal  and 
semi-diurnal  ones,  and  the  period  of  each  of  these 
new  waves  depends  on  the  duration  of  the  apparent 
revolution  of  each  of  the  attracting  bodies  round 
the  Earth.  Consequently,  for  the  Moon  there  is 
a  fortnightly  wave  and  a  monthly  wave,  and  for 
the  Sun  a  half-yearly  wave  and  a  yearly  one,  but 
their  amplitudes  are  much  less  than  those  of  the 
principal  waves. 

This  is  not  all;  we  have  seen  in  studying  the 
Earth's  movements  that  the  place  of  the  lunar 
orbit  suffers  a  recurrent  displacement,  and  that  its 
intersection  with  the  plane  of  the  terrestrial  orbit 
is  continually  being  displaced  from  west  to  east, 


292  The  Earth 

executing  a  complete  turn  in  i8J  years.  As  this 
must  be  taken  into  account  in  considering  the 
inclination  of  the  lunar  orbit  with  respect  to  the 
terrestrial  one,  it  will  readily  be  understood  that 
two  new  subsidiary  waves  are  introduced,  the  one 
having  a  period  of  i8J  years  and  the  other  a 
period  of  half  this,  but  being  of  very  small  ampli- 
tude. Other  causes  affecting  the  tides  are  the 
precession  of  the  equinoxes,  the  Earth's  polar 
displacements,  and,  in  general,  all  the  Earth's 
movements,  but  these  waves  are  negligible  in 
amplitude. 

The  complete  understanding  of  the  tides  thus 
necessitates  the  calculation  of  the  elements  of 
each  of  these  waves  separately  and  the  final  com- 
bination of  their  periodic  actions.  Lord  Kelvin 
and  Sir  George  Darwin  have  carried  out  this  work, 
using  the  beautiful  method  that  is  called  "  harmonic 
analysis,"  and,  in  France,  the  hydrographer  Matt 
has  also  made  a  careful  study  of  the  matter. 

In  order  to  represent  each  one  of  these  waves 
separately,  we  imagine  for  each  one  a  fictitious 
astronomical  body,  supposed  to  be  the  only  body 
in  the  Earth's  presence,  turning  around  the  latter 
with  a  period  equal  to  that  of  the  wave  it  re- 
presents, remaining  always  in  the  plane  of  the 
equator  and  at  a  constant  distance  from  the 


RHytHmic  Movements  of  tHe  Ocean    293 

terrestrial  globe.  This  fictitious  body,  the  mass 
and  distance  of  which  may  be  calculated  from 
astronomical  data,  therefore  always  produces  a 
constant,  unique  wave.  The  characteristics  of  as 
many  fictitious  bodies  as  there  are  elementary 
waves  to  be  combined  are  thus  calculated.  The 
list  of  the  principal  ones  is  as  follows : 

The  semi-diurnal  waves  are  first  the  mean  lunar 
wave,  which  represents  the  mean  movement  of 
the  semi-diurnal  tide.  This  is  the  most  important 
one  of  all,  and  the  fictitious  body  that  would 
engender  it  would  have  a  mass  equal  to  ^o 
of  that  of  the  Moon;  secondly,  the  sidereal  wave 
(lunar  fraction)  of  period  equal  to  a  semi-sidereal 
day.1  This  gains  on  the  first  wave  above,  which 
is  regulated  by  the  lunar  day,  and  the  two  coin- 
cide once  in  approximately  fifteen  days;  thirdly, 
the  lunar  elliptical  wave,  which  lags  behind  the 
first  by  a  quantity  equal  to  the  mean  anomalistic 
motion  of  the  Moon  in  its  orbit;  the  two  coincide 
therefore  at  intervals  of  one  month;  fourthly,  a 
mean  solar  wave;  fifthly,  a  sidereal  wave  (solar 
fraction) ;  and  sixth  a  solar  elliptical  wave,  the  last 
three  bearing  the  same  relation  to  the  Sun's  action 
as  the  first  three  do  to  the  Moon's  action. 

1  The  sidereal  day,  the  period  of  one  exact  revolution  of  the 
Earth,  is  23  hours  56  minutes  4.09  seconds  long. — Ed. 


294  The  EartH 

Next,  we  come  to  the  group  of  diurnal  waves, 
first,  a  lunar  diurnal  wave  and,  secondly,  a  sidereal 
diurnal  wave  (lunar  fraction),  thirdly,  a  solar 
diurnal  wave  and,  fourthly,  a  sidereal  diurnal  wave 
(solar  fraction)  which  represent  almost  exactly  the 
effects  of  the  respective  declinations  of  the  Moon 
and  the  Sun.  These  waves  are,  taken  two  by  two, 
of  almost  equal  magnitude,  so  that  at  the  epochs 
of  their  coincidences  the  effect  of  each  of  the  groups 
is  sensibly  doubled,  while  at  the  intermediate 
epochs  the  two  nearly  neutralise  each  other. 

To  these  astronomical  waves,  we  must  add 
others  of  different  characters.  In  the  first  place; 
there  are  meteorological  waves,  the  result  of 
regular  diurnal  and  seasonal  winds  on  the  tides; 
these  are  inseparable  from  the  diurnal  and  annual 
solar  waves.  Secondly,  there  are  the  waves  result- 
ing from  the  complications  that  the  more  or  less 
irregular  conformation  of  shores  and  depths  impose 
on  tidal  phenomena.  This  is  especially  the  case  in 
estuaries  where  the  tidal  flow  traverses  large  but 
shallow  spaces. 

Thus,  each  one  of  these  waves  (we  may  enu- 
merate sixteen  for  the  tides  on  the  coasts  of  France, 
and  twenty-one  for  the  tides  of  the  Indies)  may 
be  imagined  to  correspond  to  the  action  of  a 
fictitious  body,  the  characteristics  of  which  can 


RHytHmic  Movements  of  tKe  Ocean     295 


be  calculated.  It  now  remains  to  combine  all 
these  individual  waves  into  one  resultant  whole. 
If  this  had  to  be  done  by  calculation  only,  the 
work  would  be  of  great  length  and  extreme 
difficulty,  but  Lord  Kelvin  originated  the  clever 
and  simple  idea  of  bringing  the  aid  of  a  mechanical 
arrangement  to  the  solution  of  the  problem.  He 
constructed  an  apparatus  called  the  tide-pre- 
dicter;  when  the  particulars  of  each  of  the  com- 
ponent waves  are  known  for  any  given  place  such 
as,  for  example,  Brest,  the  tide-predicter  enables  us 
to  combine  them  all  into  a  unique,  complex,  but 
mathematically  exact  curve 
which  graphically  represents 
the  resultant  phenomenon,  that 
is  to  say,  at  the  time  repre- 
sented by  the  abscissa  of  a  point 
on  this  curve,  the  amplitude  of 
the  complex  tide-wave  which  is 
the  resultant  of  the  composition 
of  all  the  individual  wave-ele- 
ments is  represented  by  the 
ordinate  of  that  point. 

The  illustrious  English  physi- 
cist utilized,  in  the  construction 
of  this  apparatus,  the  fact  that 
the  curves  which  express  tide- 


FIG.  28. — Tracing 
of  a  Sine-Curve  by 
Combination  of  Re- 
volving Drum  and 
Rod  and  Crank 
Movement. 


296 


The   Earth 


waves  are  sine-curves,  and  that  every  sine-curve 
can  be  very  easily  traced  by  a  pencil  actuated 
by  a  rod  and  crank  movement,  the  two  trains 
of  the  crank  and  the  revolving  drum  being 
connected  (Fig.  28).  If,  therefore,  we  wish  to 
compound  a  certain  number  of  sine-curves  each 


FIG.   29. — Principle  of   the  Tide  Predicter:    combining   Six 
Elementary  Waves. 

of  which  represents  an  elementary  wave,  say,  for 
example,  six,  we  take  six  crank  rod  systems  (Fig. 
29) ;  the  length  of  each  crank  represents  the  am- 
plitude of  the  corresponding  wave;  the  velocity 
of  its  rotation  is  governed  by  the  period  of  the 
wave  in  question.  Suitable  trains  of  wheels  en- 
able each  to  be  rotated  by  a  separate  movement 
with  its  correct  individual  velocity.  The  posi- 
tion that  each  crank  occupies  in  relation  to  that 


RHytHmic  Movements  of  tHe  Ocean     297 

which  represents  the  lunar,  semi-diurnal  wave 
when  the  latter  is  vertical  shows  the  phase  or  rela- 
tive displacement  of  the  corresponding  wave. 

Each  rod  thus  rises  and  falls,  and  if  every  one 
carried  a  pencil  it  would  trace  graphically  on  a 
paper,  passing  with  a  uniform  motion,  the  sine- 
curve  which  is  the  graphical  representation  of  the 
wave  to  which  it  corresponds.  In  order  to  com- 
bine all  these  movements  to  obtain  the  total  effect, 
as  regards  both  sign  and  magnitude,  Lord  Kelvin 
furnished  each  rod  with  a  pulley.  The  rods  are 
separated  by  spaces  equal  respectively  to  the 
diameters  of  the  pulleys.  One  single  thread  passes 
through  the  grooves  of  all  the  pulleys;  it  is  fixed 
at  one  of  its  ends  and  at  the  other  end  it  carries  a 
weight  furnished  with  a  pencil.  When  the  whole 
system  is  started  working  it  will  be  obvious  that 
the  weight,  in  rising  and  falling,  represents  at 
each  instant  the  resultant  ordinate  of  each  of  the 
partial  ordinates  of  the  six  component  sine-curves. 
If  the  pencil  touches  a  cylinder  covered  with 
paper  and  turning  uniformly,  the  required  curve 
expressing  the  resultant  phenomenon  will  be 
traced  upon  the  paper.  In  reality  there  are  sixteen 
pulleys  in  the  apparatus  in  actual  use  for  hy- 
drographical  purposes  corresponding  to  sixteen 
component  waves. 


298  THe  EartH 

Such  is  the  marvellous  instrument  which  can  do 
in  a  few  minutes,  exactly  and  without  effort  on 
our  part,  what  would  necessitate  months  of  long 
calculations  to  achieve. 

Before  this  apparatus  was  invented,  and  it 
dates  back  only  a  few  years,  tide  almanacs,  which 
are  so  necessary  to  sailors,  had  to  be  prepared  in 
advance.  At  a  given  place  the  local  circum- 
stances, coasts,  estuaries,  the  sea-bottom,  etc., 
which  so  greatly  influence  the  tides,  are  the 
constants  for  that  place ;  the  only  variable  quanti- 
ties are  the  respective  positions  of  the  Earth, 
Moon,  and  Sun.  Now  every  18  years  and  II 
days  these  three  bodies  return  to  exactly  the  same 
relative  positions.  Consequently  if  the  heights 
of  the  tides  be  observed  at  a  given  place  during 
a  period  of  18  years  and  n  days  by  means 
of  the  instruments  called  tide-recorders  or  tide- 
measurers,  which  are  based  on  the  principle  of 
communicating  vessels,  we  obtain  the  values  of  the 
tide  for  each  day  of  the  following  period.  This 
eighteen  years  and  eleven  days  period  was  known 
to  the  ancients,  who  called  it  the  Saros;  seventy 
eclipses  always  occur  during  it,  forty-one  of  which 
are  of  the  Sun  and  twenty-nine  of  the  Moon,  and 
the  eclipses  observed  during  this  period  reoccur  at 
corresponding  epochs  during  the  following  period. 


RHytHmic  Movements  of  tine  Ocean     299 

The  tides  propagate  themselves  in  the  form  of  a 
wave,  and  are  consequently  similar  to  the  seismic 
waves  of  translation  of  which  we  have  spoken  in 
Chapter  VII.  Their  velocity  of  propagation  is, 
therefore,  proportional  to  the  square  root  of  the 
depth  of  the  water  at  the  surface  of  which  they  are 
travelling.  This  explains  a  very  curious  observed 
fact:  when  the  tide  arrives  at  the  west  coasts  of 
France  the  water  rises  slowly  at  first,  from  the  level 
of  low  tide,  but  the  velocity  increases  little  by  little 
and  in  proportion  as  the  level  rises  the  tide-stream 
is  accelerated,  the  incoming  current  getting  very 
strong  and  the  rate  of  rise  very  rapid.  This  is  a 
consequence  of  the  above  law  of  wave  propagation, 
for  the  depth  is  nothing  at  first,  at  the  time  of 
low  tide,  and  it  increases  in  proportion  as  the  sea 
recovers  the  sloping  parts  of  the  shore,  being 
greatest  just  at  the  hour  of  high  tide. 

A  similar  consideration  enables  us  to  understand 
why  there  are  at  certain  places,  St.  Malo,  for 
instance,  tides  reaching  a  height  of  14  metres, 
[46  ft.]  while  the  theoretical  tide  ought  not  to 
surpass  60  centimetres  [23.5  in.].  The  explana- 
tion is  that  the  tide  manifests  itself  by  an 
undulatory  movement  which  communicates  a 
considerable  vibration  velocity  to  the  molecules 
of  the  water.  So  long  as  the  wave  travels  over  a 


300  TKe  Earth 

deep  ocean,  for  example,  at  the  surface  of  depths 
of  water  of  4000,  5000,  or  even  6000  metres  [2.5, 
3,  3.75  miles]  the  velocity  of  propagation  is 
sensibly  constant.  But,  when  the  wave  of  the 
tide  approaches  the  coasts  of  Europe  it  meets  the 
continental  plateau  which  is  a  kind  of  base  or 
foundation  on  which  the  European  continent 
stands,  only  200  metres  [660  ft.]  below  the  surface 
of  the  sea.  The  force  of  the  incoming  tide  is  there- 
fore communicated  to  a  much  smaller  liquid  mass, 
and  this  results  in  the  very  high  elevation  of  the 
water  along  the  coasts,  particularly  in  the  English 
Channel,  the  narrow  form  of  which  accentuates 
the  phenomenon.  Maps  may  be  made  enabling 
us  to  follow  the  stages  of  the  arrival  of  the  tide- 
wave  by  the  tracing  of  co-tidal  lines  joining  the 
places  where  the  wave  arrives  at  the  same  time. 
The  more  the  co-tidal  lines  are  compressed  together 
the  greater  the  accentuation  of  the  tides. 

Two  English  hydrographers,  Whewell  and 
Lubbock,  have  carried  still  further  this  idea  of  the 
undulatory  transmission  of  the  tide.  According 
to  them,  in  order  that  the  phenomenon  of  the  tide- 
wave  may  be  produced,  it  must  take  rise  on  an 
illimitable  ocean  in  such  a  way  that  the  wave 
following  the  movement  of  the  attracting  body 
can  freely  make  the  entire  tour  of  the  globe.  Now 


RHytHmic  Movements  of  tHe  Ocean    301 

these  conditions  are  only  realised  in  the  southern 
ocean  in  the  vast  and  terrible  sea  which  completely 
surrounds  the  terrestrial  globe  and  is  bounded 
by  the  Antarctic  continent,  lying  between  this  and 
Cape  Horn,  the  Cape  of  Good  Hope  and  Australia. 
Whewell  and  Lubbock  believe  that  it  is  here  that 
the  generating  tide-wave  takes  rise  without  any 
continental  obstacle.  The  tide- wave  which  occurs 
in  the  Atlantic  would,  consequently,  be  only  a 
secondary  wave  derived  from  the  principal  one. 

Facts  have  been  observed  which  give  a  strong 
support  to  this  original  idea.  All  along  the 
Atlantic  coasts,  even  on  the  extreme  south  coasts 
of  the  Argentine,  are  stations  at  which  the  tides 
are  noted.  It  is,  thus,  possible  to  follow  hour  by 
hour  the  propagation  of  the  northward  travelling 
tide- wave.  Now  it  has  been  shown  that,  when  the 
tide-wave  arrives,  for  example  at  midday,  at  the 
Straits  of  Magellan,  it  reaches  Cape  Corrientes 
near  the  mouth  of  the  Rio  de  la  Plata  at  midnight, 
that  is,  twelve  hours  later.  After  another  twelve 
hours  it  gets  to  the  Canary  Isles  and  finally  twelve 
hours  later,  that  is  to  say  at  midnight  on  the  second 
day,  its  influence  is  felt  in  the  tide-recorder  at 
Brest.  It  has,  therefore,  taken  in  all  thirty-six 
hours  to  cross  the  Atlantic  from  south  to  north. 
It  can  also  be  proved  that  at  Brest  the  equinoctial 


302  The  Earth 

tide  is  only  felt  thirty-six  hours  after  the  theoretical 
moment  when  the  Moon  and  Sun,  whose  attractive 
forces  are  then  additive,  produce  the  maximum 
possible  tide.  This  is  a  very  remarkable  confirma- 
tion of  Whewell  and  Lubbock's  theory.  Never- 
theless, there  is  also  a  fact  which  runs  counter  to 
this  theory.  At  no  part  in  the  islands  of  the 
Southern  Sea,  whether  at  Kerguelen,  St.  Paul,  or 
New  Amsterdam  has  the  age  of  the  tide  been  found 
to  be  nothing  at  all,  as  it  should  be  in  accordance 
with  the  preceding  theory.  Adhuc  sub  judice  Us 
est. 

Thus,  the  phenomenon  of  the  tide  is  ruled,  as 
regards  its  amplitude,  by  coastal  configuration. 
In  Europe,  it  is  around  England  and  France  that 
the  greatest  variation  of  level  takes  place.  Land- 
locked seas,  on  the  other  hand,  such  as  the  Medi- 
terranean and  the  Baltic,  have  only  insignificant 
tides;  in  the  Gulf  of  Gabes  tides  of  a  metre  [or 
yard]  are  sometimes  observed  and  these  arise 
chiefly  from  the  Atlantic  tide  coming  through  the 
Strait  of  Gibraltar.  On  the  other  hand,  in  the 
partially  landlocked  seas  and  on  large  lakes, 
continuous  variations  of  level  are  observed,  the 
periodicity  of  which,  although  real,  has  no  as- 
tronomical cause.  These  effects  are  readily 
visible  on  the  Lake  of  Geneva.  Their  origin  is 


RHytHmic  Movements  of  tKe  Ocean     303 

probably  meteorological;  when  the  atmospheric 
pressure  distinctly  increases  at  one  end  of  an 
elongated  lake,  the  level  there  is  caused  to  fall 
and  consequently  that  of  the  other  end  rises. 
Such  a  momentary  inequality  of  the  surface  level 
leads  to  a  re-establishment  of  hydrostatic  equi- 
librium by  means  of  a  series  of  oscillations  the 
duration  of  which  depends  on  the  size  of  the  lake 
and  its  depth;  theory  and  observation  have 
always  been  in  accordance  on  this  point. 

Seas  such  as  the  ^Egean  show  also  tide  effects 
similar  to  that  of  the  Lake  of  Geneva,  and,  at  the 
epoch  of  the  equinoxes,  the  joint  effect  of  these 
and  the  small  true  tides  which  then  occur  is 
perceived.  The  variations  of  level  have  thus 
a  complexity,  more  apparent  than  real,  which 
the  preceding  considerations  now  enable  us  to 
elucidate  completely. 

The  tides,  as  we  have  said,  give  rise  to  flow-  or 
ebb-currents,  according  as  the  water  is  rising  or 
falling.  The  configuration  of  the  shore  may  be 
such  as  to  be  favourable  to  their  establishment, 
or,  on  the  other  hand,  it  may  tend  to  lessen  them. 
In  the  former  case,  they  may  attain  considerable 
strength  and  may  present,  at  certain  times, 
dangers  for  navigators.  As  cases  in  point  we 
have  the  race  at  Sein  in  Finisterre,  and  the  Blan- 


304  The  Earth 

chard  race  in  the  English  Channel,  where  during 
the  equinoctial  tides  the  velocity  of  the  current 
exceeds  eight  miles  per  hour,  and  the  whirlpools 
which  occur  in  certain  places  after  the  change  of 
the  tide,  on  account  of  the  meeting  of  two  con- 
trary currents,  such  as  the  Maelstrom  in  the 
north  of  Norway,  Corryvrekan  in  the  Hebrides, 
and  the  legendary  whirlpool  of  Charybdis,  more 
dangerous  in  the  fable  than  in  reality. 

We  shall  now  deal  with  other  rhythmic  move- 
ments exhibited  by  the  waters  of  the  sea,  viz., 
the  swell  and  the  waves, 

A  representation  on  a  small  scale  of  the  propaga- 
tion of  waves  over  the  surface  of  water  may  be 
obtained  by  letting  a  pebble  fall  into  a  basin  of 
water.  Circular  ripples  or  undulations  are  seen 
moving  outwards  from  the  point  of  immersion  of 
the  pebble  towards  the  edge  of  the  basin.  The 
same  thing  occurs  on  a  large  scale  at  the  surface 
of  the  oceans,  which  are  always  traversed  by 
undulations  of  more  or  less  importance.  Such 
a  movement  is  that  produced  in  calm  weather. 
When  the  wind  begins  to  freshen  and  rise,  the 
ridges  of  water  which  constitute  the  undulations 
of  the  swell  lose  their  beautiful  regularity;  they 
cease  to  be  symmetrical,  becoming  steep  while 


RHytHmic  Movements  of  tHe  Ocean     305 

their  surfaces  are  covered  with  ripples  and  sub- 
sidiary wavelets.  Little  by  little,  under  the 
influence  of  the  wind,  the  wave  slopes  become 
hollowed  and  the  summits  begin  to  overhang; 
finally  they  give  way  and  fall  over,  imprisoning 
a  mass  of  air  which  escapes  in  bubbles  of  whitish 
foam,  constituting  the  white-crested  waves  fami- 
liarly called  "white  horses."  These  are  breaking 
waves. 

The  height  of  waves  is  sometimes  considerable. 
While  not  reaching  the  values  of  40  and  50  metres 
[130  to  150  ft.]  which  the  assertions  of  ancient 
navigators  attributed  to  them,  on  account  of  an 
optical  error  into  which  it  is  easy  to  fall,  they 
do  actually  attain  in  the  Southern  Seas,  at  their 
highest,  15  to  16  metres  [50  to  55  ft.],  10  to  II 
metres  [35  to  40  ft.]  in  the  Indian  Ocean,  8  to  9 
metres  [30  to  35  ft.]  in  the  Atlantic,  and,  finally, 
in  the  Mediterranean  5  to  6  metres  [15  to  20  ft.], 
always  speaking  of  their  highest,  and,  when  freely 
propagated,  far  from  the  coasts. 

For,  when  a  system  of  undulation  is  not  freely 
propagated  but  meets  an  obstacle,  the  phenomenon 
is  complicated  by  that  of  interference  between 
the  direct  movement,  and  the  one  reflected  from 
the  obstacle.  In  this  way,  the  height  of  waves 
may  become  enormous.  This  is  what  happens 

30 


306  The  EartK 

when  they  beat  upon  the  coasts;  they  rise  up  to 
heights  of  40  to  50  metres  [130  to  150  ft.]  and  fall 
back  in  masses  of  foam.  The  same  thing  happens 
when  a  large  vessel  is  going  at  full  speed  in  the 
opposite  direction  to  the  movement  of  propagation 
of  the  waves;  these  dash  over  the  bow  and  may 
reach  even  to  the  highest  superstructure  when  they 
sometimes  do  damage  or  wash  away  men. 

When  several  series  of  waves,  travelling  in 
different  directions,  meet  together,  as  a  result  of 
some  special  circumstances,  interference  pheno- 
mena are  again  produced,  and  the  sea  becomes 
agitated  and  choppy.  This  occurs  at  the  centre 
of  cyclones,  where  thousands  of  undulatory  move- 
ments meet  together,  engendered  by  winds  that 
have  every  possible  direction  since  they  form  part  of 
a  whirling  movement.  In  the  Mediterranean,  the 
closed  contour  of  the  coasts  gives  rise  to  reflected 
movements  in  all  directions,  and  consequently  the 
sea  presents  short  and  choppy  waves  which  often 
render  navigation  difficult,  although  it  is  not 
actually  very  agitated. 

The  length  of  waves  between  consecutive 
summits  is  about  20  to  30  times  their  height;  the 
great  waves,  15  metres  [50  ft.]  in  height,  of  the 
Southern  Seas  may  thus  reach  lengths  of  300  to 
450  metres  [325  to  500  yds.].  Consequently,  the 


RHytHmic  Movements  of  tHe  Ocean     307 

slope  of  these  liquid  heights  is  quite  a  gentle  one, 
and  this  circumstance  is  a  fortunate  one,  for, 
without  this,  the  falling  over  of  the  crests  when 
they  break  would  render  all  navigation  impossible. 
If  only  a  superficial  examination  be  made,  it 
seems  that  when  undulations  of  water  are  caused 
in  a  basin,  the  water  itself  is  transported  towards 
the  edge  of  the  basin.  More  attentive  observation 
will,  however,  show  that  this  is  not  the  case,  for  a 
small  piece  of  wood  thrown  on  the  surface  of  the 
water  rises  and  falls  alternately  with  the  passage 
of  the  waves,  but  does  not  move  any  nearer  to  the 
edge.  The  molecules  of  the  liquid,  therefore,  move 
up  and  down  in  one  place ;  two  German  physicists, 
the  brothers  Weber,  have  experimentally  studied 
the  matter  and  have  found  as  a  result  that  each 


FIG.   30. — Circular   Vibratory  Motion  of  a   Molecule  of  a 
Liquid  (Formation  and  Propagation  of  a  Swell). 

aqueous  molecule  describes  a  closed  curve  (Fig. 
30),  and  that  it  is  the  combination  of  these  vibra- 
tory movements,  transmitted  from  molecule  to 
molecule  which  constitutes  the  propagation  of 
undulatory  movements.  In  proportion  as  the 
molecule  in  question  lies  deeper  under  the  water, 


308  The  Earth 

the  circular  path  described  by  it  is  flatter  and  so 
becomes  a  more  and  more  flattened  ellipse  which, 
ultimately,  is  reduced  to  a  straight  line.  At  great 
depths,  therefore,  wave  propagation  takes  place 
by  a  simple  rectilinear  horizontal  movement,  for- 
ward and  backward,  of  the  molecules  of  the  liquid. 
Experiment  has  shown  that  surface  agitations 
make  themselves  felt  down  to  a  depth  of  300  to 
350  times  the  height  of  the  undulations  produced; 
there  is,  thus,  a  level  below  which  superficial 
agitation  is  practically  not  transmitted  at  all. 

The  most  simple  manifestation  of  undulatory 
movement  at  the  surface  of  the  sea  is  the  swell 
characterised  by  the  absence  of  that  white  foam 
which  sailors  call  "white  horses."  It  forms  at 
the  surface  of  the  sea  regular  ridges,  with  regu- 
larly curved  sides  and  which  move  majestically 
over  the  water  when  the  atmosphere  is  calm. 

The  undulation  of  the  swell  is  in  profile  the 
curve  above  represented  and  is  called  by  mathe- 
maticians a  cycloid.  Such  an  undulation  is 
characterised  by  its  length,  by  which  is  meant  the 
constant  distance  between  two  consecutive  crests; 
by  its  velocity  of  propagation,  which  is  the  distance 
traversed  in  a  second  by  the  condition  of  undula- 
tion; by  the  period,  which  is  the  time  taken  for 
one  crest  to  succeed  the  next,  and,  finally,  by  the 


RHytHmic  Movements  of  tHe  Ocean     309 

amplitude  or  height  of  the  undulations,  that  is  to 
say,  by  the  vertical  distance  between  the  crest 
and  the  hollow  of  the  wave. 

The  movements  of  the  sea  represent  the  produc- 
tion of  a  considerable  sum  of  mechanical  energy. 

Considering  first  the  waves,  their  velocity  of 
propagation  is  about  twenty-five  marine  miles 
per  hour,  that  is,  more  than  45  kilometres  [28 
standard  miles].  Furthermore,  a  large  mass  of 
water  is  contained  in  a  wave  10  metres  [35  ft.], 
high,  and,  as  the  energy  of  the  wave  is  its  mass 
multiplied  by  the  square  of  the  velocity,  it  so 
attains  a  considerable  amount.  A  wave  of  the 
height  and  velocity  just  mentioned  develops 
about  two  thousand  horse-power  per  metre  [or 
yard]  width.  Stephenson  has  also  measured 
directly  the  force  exerted  on  a  given  surface  by 
the  shock  of  such  waves  and  finds  it  thirty  tons 
per  square  metre  [French  ton  =2204.62  Ibs.].  If 
we  remember  that  a  like  force  is  produced  every 
ten  or  fifteen  seconds,  this  being  approximately 
the  period  of  these  waves,  for  periods  of  some  days, 
it  is  obvious  that  they  represent  a  large  amount  of 
energy. 

These  powerful  masses  of  water  beat  against  the 
coasts,  and  by  long-continued  attack  wear  away 
the  rocks  and  break  off  portions  of  them.  In  this 


310  The  EartH 

way  the  granites  of  Brittany  are  scooped  and 
hollowed,  and  the  chalk  cliffs  of  Normandy  are 
undermined  from  their  base.  Similarly  defensive 
works  and  breakwaters  that  man  has  erected,  at 
the  cost  of  years  of  difficult  labour,  to  protect 
harbours  may  sometimes  be  broken  through  and 
destroyed  by  a  single  storm  in  a  few  minutes. 
Perhaps,  at  some  future  time,  it  may  be  possible 
to  harness  these  hitherto  unutilised  forces;  then 
transmitted  inland  by  means  of  electric  currents, 
the  movement  of  the  waters  of  the  sea  could  be 
put  to  use  instead  of,  as  at  present,  only  causing 
destructive  effects. 

As  regards  the  tides,  the  power  represented  by 
the  alternate  rising  and  falling  of  the  level  of  the 
sea  is  also  considerable,  and  would  be  easier  to 
utilise;  it  would  suffice  to  construct  vast  basins 
which  could  actually  be  done  in  many  cases  by 
closing  in  an  estuary  by  means  of  a  dam,  forming 
a  natural  reservoir  which  would  be  filled  at  high 
tide  by  the  flowing  in  of  the  sea.  The  opening 
could  then  be  shut,  and  the  water  thus  maintained 
at  the  high  level  would  work  turbines  in  rushing 
down  when  the  level  of  the  water  outside  had 
fallen ;  consequently  the  power  would  be  available. 
In  regions  such  as  that  of  St.  Malo,  where  the 
tides  reach  a  height  of  15  metres  [50  ft.]  at  the 


RHytHmic  Movements  of  tHe  Ocean     311 

equinoxes  there  would  be  an  ample  reserve  of 
energy.  In  the  bay  of  Mont  St.-Michel  each 
square  kilometre  [.38  sq.  mile]  of  the  sea  surface 
represents  an  average  force  of  20,000  horse-power, 
and  the  bay  is  not  less  than  300  square  kilometres 
[116  sq.  miles]  in  area.  If,  therefore,  it  was  closed 
by  an  embankment  we  should  have  available  about 
6,000,000  horse-power,  and  the  work  would  not 
be  more  difficult  than  the  making  of  the  Suez 
Canal  or  of  a  railway  across  the  Sahara.  And 
a  number  of  other  bays  would  lend  themselves 
to  similar  procedure.  The  damming  of  the 
Ranee  would  also  give  more  than  200,000  available 
horse-power. 

This  enormous  quantity  of  energy  is  produced 
by  the  periodic  attractions  of  the  Sun  and  Moon 
particularly  of  the  latter.  This  seemingly  dead 
world,  therefore,  gives  rise  to  movement  and  force 
upon  the  Earth's  surface.  This  is  a  beautiful 
example  of  the  rejuvenation  of  everything,  and  of 
that  evolution  which  we  recognise  everywhere  in 
the  study  of  organised  matter.  As  has  been  so 
truly  said,  "Life  is  reborn  out  of  death,"  and  it 
seems  probable  that  this  applies  to  the  life  of 
worlds  also. 


CHAPTER  X 

THE    CIRCULATION    OF    THE    EARTH,    MARINE    AND 
ATMOSPHERIC 


of  the  characteristics  of  life  is  a  continu- 
ous  circulation  in  the  body  of  the  living 
being;  animals  and  plants  have  such  a  circula- 
tion inseparable  from  their  very  existence.  If  we 
consider  that  the  Earth  "lives"  and  evolves  it 
also  ought  to  have  its  own  circulation. 

We  have  already  seen  that  an  electric  circula- 
tion exists  in  the  crust  and  its  nucleus,  and  we  have 
mentioned  the  convection  movements  which  occur 
in  the  liquid  superficial  mass  of  its  interior  magma. 
We  shall  find  in  the  course  of  this  chapter  that  the 
two  media  which  envelop  it,  the  hydrosphere  and 
the  atmosphere,  are  both  the  seat  of  a  continuous 
circulation,  the  importance  of  which,  from  the 
point  of  view  of  the  exterior  aspect  of  the  ter- 
restrial relief,  is  very  great.  We  shall  first  consider 
the  atmospheric  circulation  and  then  the  oceanic 
circulation,  and  we  shall  see  that  these  two  pheno- 

312 


.AtmospHeric  Circulation  313 

mena  are  connected  by  such  a  direct  relationship 
that  they  are  inseparable  from  each  other. 

Although  the  atmosphere  does  not  possess  the 
fixity  of  composition  of  a  chemical  compound, 
yet,  at  any  rate  in  its  lower  layers,  the  composi- 
tion is  found  to  be  very  nearly  invariable,  viz.: 
21%  of  oxygen,  78%  of  nitrogen,  and  i%  of  argon, 
besides  infinitesimal  traces  of  other  rare  gases, 
such  as  xenon,  neon,  and  krypton  and  also  of 
hydrogen  and  helium.  This  percentage  composi- 
tion is  by  weight.  We  speak  here  only  of  the 
simple  gases  that  are  chemical  elements  in  the  ac- 
cepted use  of  the  term;  that  is,  chemical  elements 
in  the  sense  in  which  the  word  was  used  prior  to 
the  discovery  of  radioactive  phenomena,  and  in 
which  it  is  still  used  to  elements  in  the  customary 
sense  of  the  word,  whether  they  may  be  transformed 
into  simpler  elements  by  radioactive  process  or 
not.  Two  of  the  gaseous  compounds,  carbonic 
acid  and  water-vapour,  play  an  important  part 
in  the  economy  of  our  Earth.  In  speaking  of  the 
first  stages  of  the  Earth's  existence,  we  have 
dwelt  on  the  protective  function  which  they  fulfil 
in  regard  to  the  surface  temperature  of  the  Earth, 
constituting  a  thermal  mantle.  The  remaining 
ones  such  as  ammonia,  nitrogen,  and  sulphur  com- 
pounds, ozone,  etc.,  are  present  in  only  very  small 


314  The  Earth 

and  variable  quantities.  The  lower  layers,  in 
contact  with  the  land  and  the  seas,  contain  almost 
the  same  proportion  of  simple  gases.  The  pro- 
portion of  hydrogen  and  helium  increases  with 
height,  and  in  the  highest  regions  of  the  terrestrial 
atmosphere,  the  little  air  which  remains  is  com- 
posed of  99%  of  hydrogen  and  l%  of  helium. 

These  gaseous  substances  are  subjected  to  two 
forces,  first,  the  centrifugal  force,  in  consequence 
of  the  Earth's  rotation,  and  secondly,  the  Earth's 
attraction.  Furthermore,  they  are  constantly 
exposed  to  the  thermal  action  of  the  solar  radia- 
tion. Water- vapour  is  the  best  absorbent  of 
these  rays  and  so  constitutes  the  principal  agent 
in  the  heating  of  the  air  under  the  influence  of  the 
Sun's  rays. 

If  the  terrestrial  globe  were  a  surface  without 
relief,  wholly  covered  with  a  homogeneous  sub- 
stance such,  for  example,  as  sand,  and  if  its  axis 
of  rotation  were  perpendicular  to  the  plane  of  its 
orbit  instead  of  being  inclined,  every  point  on  the 
Earth  would  be  subject  to  constant  temperature 
conditions,  save  for  the  little  variations  in  the 
distance  of  the  Earth  from  the  Sun  in  accordance 
with  the  law  of  Kepler.  For  every  place  on  the 
globe  throughout  the  entire  year  the  days  and 
nights  would  have  an  equal  duration  of  twelve 


Atmospheric  Circulation  315 

hours.  The  temperature,  which  would  be  at  a 
maximum  in  the  region  of  the  equator,  upon  which 
the  Sun's  rays  would  fall  normally,  would  diminish 
regularly  up  to  the  poles,  where  these  rays,  grazing 
the  surface,  would  have  no  heating  power  at  all, 
since  this  depends  on  the  sine  of  the  obliquity. 
Consequently,  there  would  be  no  seasons,  and 
the  different  terrestrial  climates,  that  is  to  say, 
the  sum-total  of  meteorological  conditions  at  each 
point  of  the  globe,  would  vary  from  one  place  to 
another  in  a  continuous  way. 

But  in  reality  things  are  not  nearly  as  simple  as 
this.  In  the  first  place  the  axis  of  rotation  of  the 
Earth  is  not  erect;  the  plane  of  the  terrestrial 
equator  makes  an  angle  of  23^°  with  the  plane 
of  the  ecliptic.  From  the  resultant  inclination, 
and,  therefore,  the  existence  of  seasons  and  the 
inequality  of  the  days  and  nights,  the  Earth  is 
divisible  into  Geographical  Zones :  first,  the  Torrid 
Zone,  through  the  centre  of  which  the  equator 
passes,  and  which  comprises  all  places  between 
the  two  tropics,  those  circles  of  latitude  correspond- 
ing to  23^°  North  and  South  latitude  respectively  ; 
secondly,  the  two  Frigid  Zones,  the  centres  of 
which  are  occupied  by  the  respective  poles,  com- 
prising the  regions  between  the  pole  and  the  lati- 
tude circle  of  66y£°  North  or  South  respectively; 


316  The  Earth 

lastly,  the  two  Temperate  Zones,  which  include 
all  the  regions  between  the  Frigid  Zones  and  the 
Torrid  Zone. 

Another  factor  making  for  complexity  in  the 
superficial  heating  of  the  Earth  is  the  lack  of 
homogeneity  of  its  surface  which  is  covered  with 
water  over  nearly  three-fourths  of  its  area,  while 
the  dry  part  is  characterised  by  a  varied  relief 
including  mountains  and  valleys,  high  plateaux, 
depressions,  and  deserts.  The  nature  of  the  soil, 
and  therefore  its  absorbing  power  for  heat  radia- 
tion, varies  from  one  place  to  another.  Conse- 
quently, the  heating  of  the  atmospheric  layer  resting 
upon  the  soil  will  thus  also  vary  from  place  to  place. 

As  regards  the  oceans,  which  form  the  chief 
part  of  the  Earth's  surface,  the  matter  is  simpler, 
for  their  surface  is  homogeneous  and  without 
relief.  The  molecules  of  the  fluid  masses,  water 
and  air,  can  thus  freely  obey  the  laws  which  govern 
them — that  is  to  say,  those  of  attraction,  centrifugal 
force,  and  equilibrium  of  gaseous  substances.  We 
should,  therefore,  find  regular  atmospheric  con- 
ditions established  over  the  great  oceans,  such  as 
the  Atlantic,  the  Indian  Ocean,  the  Southern  Seas, 
and  especially,  the  Pacific.  Now  this  is  what 
actually  occurs  and  we  shall  commence  by  saying 
a  few  words  as  to  these  conditions. 


.AtmospHeric  Circulation  317 

The  tropical  regions,  viz. :  those  which  belong  to 
the  Torrid  Zone,  are  those  most  exposed  to  the 
solar  radiation,  for  twice  annually  the  Sun  passes 
through  the  zenith  of  each  of  the  places  in  this 
zone,  at  the  moment  when  it  is  true  noon  at  the 
place  in  question.  The  rays,  therefore,  fall  per- 
pendicularly on  to  the  ground,  and  so  have  the 
maximum  possible  heating  effect.  As  a  conse- 
quence, especially  along  the  equator,  the  atmo- 
sphere in  contact  with  a  warmer  substratum  is 
heated  to  the  greatest  degree  in  its  lower  layers. 
Furthermore,  it  is  the  lower  layers  which  contain 
the  most  water-vapour  and  dust  particles,  and  so 
absorb  more  completely  the  heat  radiated  from 
the  Sun.  There  are,  thus,  two  reasons  why  the 
equatorial  atmosphere  is  relatively  strongly  heated. 
By  means  of  instruments  called  bolometers  the 
quantity  of  heat  thus  received  annually  by  the 
equatorial  belt  has  been  measured.  It  has  been 
found  that  it  is  sufficient  to  vaporise  a  layer  of 
water  four  metres  [12.2  ft.]  deep  covering  the  same 
area.  Now  meteorologists,  on  the  other  hand, 
have  determined  by  means  of  observations  ex- 
tending over  numerous  years  the  average  yearly 
quantity  of  rain  which  falls  on  the  equatorial 
belt;  it  is  represented  by  a  layer  of  water  two 
metres  [7.5  ft.]  in  depth.  Even  admitting  that  the 


3i8  TKe  EartK 

whole  of  this  water  was  vaporised  by  the  solar 
heat,  and  that  none  of  it  soaked  into  the  soil  on 
which  it  fell,  it  is  obvious  that  after  complete 
vaporisation  there  would  remain  a  surplus  quan- 
tity of  heat  sufficient  to  vaporise  as  much 
again. 

Nothing  is  ever  lost  in  Nature's  admirable 
economy.  This  surplus  heat  is  employed  for 
some  other  purpose,  and  that  purpose  is  the  fur- 
ther heating  of  the  lower  layers  of  the  equatorial 
atmosphere.  Thus  superheated,  and  consequently 
of  less  density  like  the  air  in  a  fire  balloon,  these 
lower  layers  rise  upwards  to  considerable  heights. 
Also,  this  process  is  continuous,  since  the  cause 
of  the  convection  current,  viz.:  the  equatorial 
heating,  is  always  continuous.  This  is  the  fun- 
damental movement  of  vertical  circulation  in  the 
terrestrial  atmosphere,  and  it  gives  rise  to  others, 
for  in  consequence  of  the  uprising  of  masses  of 
warm  air  towards  the  upper  atmosphere,  there  is 
a  rarefaction  near  the  surface  of  the  ground,  with 
the  result  that  masses  of  cold,  dense  air  from  the 
Temperate  and  Frigid  regions  flow  towards  the 
equator  to  take  the  place  of  that  which  has  left 
it.  If  the  Earth  were  at  rest  there  would  thus 
be  northerly  winds  in  the  Northern  Hemisphere 
and  southerly  winds  in  the  Southern  Hemisphere 


Atmospheric  Circulation  319 

blowing,  in  the  direction  of  the  meridians,  towards 
the  equatorial  regions. 

But  the  Earth  is  not  at  rest;  its  movement  of 
rotation,  as  we  have  seen,  produces  a  deviation  of 
the  path  of  any  moving  body  in  the  Northern 
Hemisphere  towards  the  right,  and  in  the  Southern 
Hemisphere  towards  the  left.  These  winds,  which 
in  the  imaginary  case  flowed  in  the  north-south 
direction,  are  actually  deviated  and  become  north- 
easterly winds  to  the  north  of  the  equator  and 
south-easterly  winds  to  the  south  of  that  line. 
These  are  the  trade  winds,  which  carried  the  cara- 
vels of  Christopher  Columbus  to  the  New  World. 
To-day,  the  track  of  these  regular  winds  is  known ; 
Maury,  the  father  of  Oceanography,  was  the  first 
to  draw  up  monthly  maps  which  indicated  to 
sailors  routes  shortening  by  half  the  duration  of 
long  voyages  made  by  sailing  vessels. 

As  regards  the  ultimate  destination  of  the 
masses  of  warm  air  which  leave  the  equator  and 
rise  into  the  upper  atmosphere,  they  travel  towards 
the  poles  and  gradually  sink  down  as  they  cool, 
replacing  the  air  of  the  Temperate  and  Frigid 
regions  which  has  moved  towards  the  equator. 
These  winds  are  called  the  anti-trade  winds.  Their 
existence  is  rendered  manifest  by  the  movement 
of  the  cirrus  clouds,  which  are  in  the  form  of  deli- 


320  The  Earth 

cate  filaments  and  are  more  elevated  than  any 
other  kind  of  cloud  formation.  Cirri  always 
travel  from  the  south-west  to  the  north-east,  in 
the  Northern  Hemisphere,  driven  by  the  upper 
returning  air  currents. 

These  anti-trade  winds,  which  are  also  deviated 
by  the  Earth's  rotation,  gradually  become  westerly 
winds  and  finally  merge  into  a  current  of  air  turn- 
ing around  the  poles,  adding  their  velocity  of  pro- 
gression to  the  velocity  which  the  Earth's  rotation 
would  impart  to  their  molecules  if  they  were 
originally  at  rest.  The  masses  of  air  that  they 
displace  will  thus  turn  around  the  poles  with  a 
velocity  greater  than  that  of  the  Earth  itself,  and 
a  considerable  centrifugal  force  is  thus  produced 
tending  to  throw  the  air  outwards  from  the  axis 
of  rotation.  Consequently,  in  the  neighbourhood 
of  this — that  is  to  say,  around  the  poles,  there  is 
a  rarefaction  or  atmospheric  depression,  this  time 
of  mechanical  origin. 

Thus,  there  is  a  thermal  depression  at  the 
equator  and  a  mechanical  one  at  the  poles;  be- 
tween these  two  minima  the  principle  of  conti- 
nuity necessitates  the  existence  of  a  maximum 
of  pressure.  By  calculation,  this  should  occur 
at  latitude  30°  North  and  South,  and  observa- 
tion confirms  the  permanent  existence  of  this 


Marine  Circulation  321 

high-pressure  condition  over  the  oceans  in  these 
latitudes. 

We,  therefore,  have  a  complete  circulatory 
motion  of  the  air  masses  enveloping  the  Earth: 
the  direct  current  from  the  Pole  to  the  equator 
along  routes  inclined  to  the  north-south  direction 
on  account  of  the  Earth's  rotation,  and  the  return 
current  in  the  upper  regions  of  the  atmosphere 
from  the  equator  towards  the  poles. 

The  trade  winds  blow  continuously,  since  their 
cause,  the  solar  heating,  is  continuous.  They 
gradually  produce  a  similar  movement  of  the 
molecules  of  water  at  the  surface  of  the  sea,  since 
neither  air  nor  water  being  perfect  fluids,  friction 
is  exercised  between  their  respective  molecules; 
along  the  equator  the  molecules  of  the  water 
being  influenced  in  two  ways  at  the  same  time, 
viz. :  by  the  north-east  trade  winds  of  the  Northern 
Hemisphere  and  the  south-east  trade  winds  of  the 
Southern  Hemisphere,  follow  the  resultant  direc- 
tion of  these  movements  which  is  from  east  to 
west.  Limiting  ourselves  to  the  current  travers- 
ing the  North  Atlantic  Ocean,  the  direction  is 
from  the  coast  of  West  Africa  to  that  of  Brazil. 
This  is  the  origin  of  the  equatorial  current  which 
meets  Cape  St.  Roque  and,  because  of  the  form 
of  the  American  coast,  there  divides  into  two 

ex 


322  The  Earth 

parts.  One  branch  goes  towards  the  south, 
which  we  will  pass  over  for  the  time  being;  the 
other,  which  we  shall  follow  up,  goes  northwards 
along  the  coast  of  Guiana. 

One  portion  cf  this  branch,  always  composed  of 
very  warm  water,  passes  outside  the  chain  of  the 
Antilles  and  then  along  the  American  coast,  and 
being  deflected  towards  the  right  by  the  Earth's 
rotation  traverses  the  Atlantic  Ocean  in  a  slant- 
wise direction  from  south  to  north.  The  other 
part  enters  the  Gulf  of  Mexico  and  accumulates 
there  tinder  the  thrust  of  the  water  which  continues 
to  flow  in,  the  Gulf  being  almost  closed.  There 
it  bathes  shores  heated  by  the  tropical  sun,  and 
the  temperature  of  the  water  consequently  in- 
creases. Under  the  influence  of  the  mass  of  water 
from  the  equatorial  current  that  is  continually 
entering  the  bay,  this  heated  water  leaves  it  by 
the  only  possible  exit,  viz. :  the  Strait  of  Florida, 
through  which  it  escapes  with  a  velocity  of  4>£ 
knots  per  hour,  or  in  other  words  about  eight 
kilometres.  It,  thus,  enters  the  Atlantic  again, 
and  rejoins  and  reinforces  the  first  northward- 
moving  branch,  giving  to  that  current  additional 
mass,  greater  velocity,  and  a  higher  temperature. 

This  current  is  called  the  Gulf-Stream,  and  it 
constitutes  a  river  of  warm  water  flowing  between 


Marine  Circulation  323 

two  banks  of  cold  water,  as  Maury  has  described 
it.  On  leaving  the  Gulf  of  Mexico,  its  depth  is 
about  400  metres  [1300  ft.]  and  its  breadth  60 
kilometres  [37  miles].  In  the  latitude  of  Cape 
Hatteras,  its  depth  is  not  more  than  300  metres 
[1000  ft.],  but,  on  the  other  hand,  it  is  larger,  and 
the  width  of  its  surface  reaches  120  kilometres 
[75  miles].  It  supplies  33,000,000  cubic  metres 
[8720  millions  of  gallons]  per  second — that  is  to 
say,  2000  times  more  water  than  the  Mississippi 
at  its  outlet,  by  means  of  which  the  ancient  geo- 
graphers formerly  but  erroneously  sought  to  ex- 
plain its  origin.  These  warm  waters  carry  along 
an  enormous  quantity  of  heat.  This  quantity 
has  been  calculated  and  is  expressed  by  39,500,- 
000,000,000,000,000  calories *  daily.  This  is  equal 
to  the  whole  of  the  heat  which  falls  on  one  of  the 
Frigid  Zones  during  the  six  months  when  it  is 
lighted  and  warmed  by  the  Sun. 2 

This  warm  current  forms  the  beginning  of  an 
oceanic  circulation  which  completes  itself  by  cold 
return  currents,  serving  to  replace  the  water 


1 A  calorie  is  the  amount  of  heat  required  to  raise  the  temper- 
ature of  one  gram  [15.432  gr.]  of  water  through  i°  C.  [1.8°  F.]. 
—Ed. 

3  A  vivid  comparison  is  that  the  Gulf -Stream  transports  as 
much  heat  as  a  stream  of  molten  iron  the  size  of  the  Mississippi 
River.— Ed. 


324  The  Earth 

which  left  the  equatorial  regions  when  warmed, 
and  which  becomes  finally  cooled  near  the  poles. 
The  most  important  of  these  cold  currents  is  that 
of  Labrador,  which  comes  down  the  Baffin  Sea 
and  follows  the  coast  of  North  America,  the  climate 
of  which  is  the  result  of  the  Labrador  current  and 
is  consequently  very  cold  in  winter.  The  current 
then  plunges  under  the  Gulf-Stream  and  reappears 
at  the  surface  of  the  Atlantic  again  near  the  coast 
of  Africa,  thus  favouring  the  abundance  of  fish 
along  the  African  coast  in  the  neighbourhood  of 
the  Walfish  Bay,  by  cooling  the  high  temperature 
of  the  sea  in  that  part  of  the  Atlantic. 

Another  cold  current  also  descends  along  the 
east  coast  of  Greenland,  which  is  always  fringed 
by  pack-ice,  rendering  it  inaccessible.  The  west 
coast  of  Greenland  is  bathed  by  a  branch  current 
derived  from  the  Gulf-Stream,  and  is  open  to  sail- 
ors during  several  months  in  the  year;  the  Danes 
have  established  settlements  on  this  coast.  Float- 
ing wood  from  the  tropics  reaches  as  far  as  Disko 
Island,  plainly  showing  that  a  current  of  equatorial 
origin  has  brought  it  there.  It  brings  to  the  north- 
ern regions  of  the  Atlantic,  where  the  waters  are 
always  several  degrees  warmer  than  those  which 
surround  them,  an  enormous  quantity  of  water- 
vapour  which  causes  the  persistent  fogs  which 


Marine  Circulation  325 

occur  over  Iceland,  Newfoundland,  and  the  neigh- 
bouring ocean.  These  fogs  lie  in  the  path  of 
transatlantic  liners  during  the  winter,  and  con- 
stitute a  serious  and  permanent  danger  to  naviga- 
tion between  Europe  and  America  and  also  to  the 
fisheries. 

Such,  in  its  broad  outlines,  is  the  oceanic  cir- 
culation of  the  North  Atlantic.  An  analogous 
case  is  found  in  the  South  Atlantic,  which  pos- 
sesses a  circulation  of  a  similar  kind.  In  the  North 
Pacific  there  exists  an  important  current,  the 
Kuro-Siwo,  the  "black  river"  of  the  Japanese, 
which  although  less  rapid  and  less  warm  than  the 
Gulf -Stream  presents  in  its  entirety  the  same 
characteristics.  The  South  Pacific  and  the  Indian 
Ocean  have  also  their  circulation,  that  of  the 
Southern  Hemisphere  being  always  the  inverse 
sense  of  rotation  to  that  of  the  Northern  one. 

Finally,  in  the  southern  seas  all  the  southerly 
branches  of  the  circulation  of  the  three  great 
oceans,  South  Pacific,  South  Atlantic,  and  Indian 
Ocean,  give  a  tangential  impulsion  to  the  liquid 
masses,  and  so  thrust  them  eastwards  in  a  general 
movement,  often  accentuated  by  the  anti-trade 
winds,  which  in  these  latitudes  are  low  down  and 
blow  also  from  west  to  east.  The  Antarctic  Ocean 
is  thus  characterised  by  an  exclusively  west-east 


326  The  Earth 

direction  both  as  regards  the  movement  of  the 
water  and  by  the  air  which  surrounds  them. 

Thus,  there  is  a  great  superficial  oceanic  circu- 
lation and  also,  doubtless,  a  vertical  oceanic  cir- 
culation maintaining  an  interchange  of  water 
between  the  warm  upper  layers  of  equatorial  origin 
and  the  deep  waters,  of  polar  origin,  occupying 
the  great  depths.  Below  6000  metres  [3.75  miles] 
the  temperature  of  the  water  at  the  bottom  of  the 
seas  is  always  between  o°  C.  [32°  P.]  and  i°  C. 
[33-8°  PJ. 

Even  the  polar  regions  themselves,  where  the 
sea  is  covered  over  with  ice-fields,  are  not  exempted 
from  the  general  law  of  the  circulation  of  water. 
The  currents  traversing  these  regions  displace 
the  ice  itself  and  it  was  by  means  of  this  ice-drift 
that  Nansen,  voluntarily  imprisoning  his  ship, 
was  able  to  effect  his  journey  to  the  neighbourhood 
of  the  North  Pole.  The  mountains  of  floating 
ice,  the  icebergs,  which  are  fragments  from  the 
glaciers  covering  the  circumpolar  lands,  are  car- 
ried by  cold  currents  even  as  far  south  as  the 
regions  where  transatlantic  liners  cross.  They 
finally  disappear  in  the  warm  waters  of  the  Gulf- 
Stream,  after  constituting  a  great  danger  to 
navigation  in  the  course  of  their  drift. 

The  temperature  difference  between  equatorial 


Marine  Circulation  327 

waters  and  the  cold  waters  of  the  polar  regions 
also  causes  a  flow  of  water  from  the  Pole  to  the 
Equator,  because  of  the  resulting  difference  of 
density.  In  that  way  is  produced  a  superficial 
oceanic  circulation  which  in  magnitude  and  direc- 
tion enhances  the  circulation  that  is  primarily 
due  to  the  trade  winds.  There  is,  thus,  a  certain 
relation  between  the  aerial  and  the  marine  cir- 
culation. We  shall  see  later  that  this  relation  is 
still  more  direct,  and  that  marine  currents,  pro- 
duced by  the  movement  of  regular  winds,  react 
in  turn  upon  the  aerial  currents,  and  produce,  by 
a  wonderful  interconnection,  the  general  circulation 
of  the  atmosphere,  even  above  the  continents. 

If  we  look  at  a  map  of  ocean  currents,  it  is 
apparent  that  the  general  circuits  of  oceanic  cir- 
culation lie  around  the  regions  of  high  pressure 
which  exist  over  the  great  oceans  in  the  latitudes 
of  30°.  Above  these  warm  currents  are  masses 
of  air  to  which  the  current  communicates  both 
part  of  its  heat  and  part  of  its  movement. 

Let  us  consider  in  particular  the  Gulf-Stream; 
it  engenders  above  it  an  aerial  "  Gulf  -Stream " 
which  is  warm  and  humid,  being  rich  in  water- 
vapour,  since  its  elevated  temperature  enables  it 
to  contain  a  greater  quantity.  When  the  marine 
Gulf-Stream  meets  the  continental  plateau,  and 


328  TKe  Earth 

then  the  shores  of  the  European  continent,  it  is 
checked  by  the  obstacle  and  compelled  to  alter 
its  route,  but  the  atmospheric  stream  above  it  is 
not  so  stopped  and  continues  its  direction  unal- 
tered. The  warm  and  moist  air  masses  consti- 
tuting it  first  meet  the  western  shores  of  Europe 
and  bring  to  these  the  warmth  which  helps  to 
produce  their  pleasantly  temperate  climate,  and 
the  humidity  which  leads  to  their  actual  rainfall 
conditions.  Always  deviated  to  the  right  by  the 
Earth's  rotation,  they  condense  their  vapour  over 
Sweden,  Finland,  and  Russia,  the  great  lakes  of 
which  are  thus  fed,  then  over  the  Ural  Mountains, 
and  down  across  the  steppes  and  deserts  of  Central 
Asia.  The  warmth  and  humidity  has  been  lost 
in  passing  over  Europe,  and  it  is  as  dry  winds  that 
they  pass  over  Asia  in  completing  their  return 
journey  towards  the  equator.  Now  when  a 
country  is  swept  by  dry  winds  only,  it  never  rains 
there,  and  consequently  vegetation  cannot  flour- 
ish. Hence  the  deserts  which  mark  the  route 
of  this  atmospheric  return  current,  the  desert  of 
Turkestan,  the  desert  of  Arabia,  and  the  Sahara 
Desert.  Thus,  by  a  very  curious  reciprocal 
effect  the  Gulf-Stream  is  indirectly  the  cause  of 
the  desert-forming  climates  of  the  Old  World. 
It  also  produces  many  other  results.  As  we 


Marine  Circxilation  329 

have  already  said,  it  produces  an  atmospheric 
stream  over  it  which  passes  on  and  circulates  over 
the  land  mass  of  the  Old  World.  But  this  air 
current  is  subject  to  the  laws  which  govern  all 
gaseous  currents.  In  particular  we  know  that 
when  a  chimney  is  in  draught  the  pressure  is 
always  less  great  in  the  interior  than  outside; 
there  is  always  more  or  less  of  a  rarefaction  in  the 
central  line  of  an  air  current,  and  the  rarefaction 
is  greater  in  proportion  as  the  current  is  more 
rapid.  Thus,  all  along  the  course  of  the  atmo- 
spheric stream  we  observe  the  rotatory  storms, 
known  as  cyclones  or  cyclonic  depressions,  which 
constitute  the  storms  of  wind  and  rain  which 
come  upon  us  in  Europe  nearly  always  from  the 
westwards.  It  is  for  this  reason  that  English 
sailors  have  called  the  Gulf-Stream  the  father  of 
storms.  We  thus  see  what  action  the  marine 
circulation  exerts  upon  the  circulation  and  the 
vicissitudes  of  the  atmosphere,  even  over  the 
continents  far  away  from  the  sea. 

What  we  have  described  in  the  North  Atlantic 
region  with  regard  to  the  Gulf-Stream  applies 
equally  to  the  North  Pacific,  with  the  Kuro-Siwo. 
This  marine  current  determines  the  formation  of 
an  atmospheric  current  above  its  tepid  waters, 
which  envelops  it  and  travels  with  it.  Just  as 


330  TKe  Earth 

in  the  case  of  the  Gulf-Stream,  its  course  is  in  the 
same  sense  of  rotation  as  that  of  the  hands  of  a 
watch.  Similarly  also,  in  the  Southern  Hemi- 
sphere, the  three  warm  currents  give  rise  to  three 
aerial  currents  travelling  above  them,  both  turn- 
ing in  the  contrary  direction  to  the  hands  of  a 
watch. 

The  consideration  of  these  atmospheric  circuits 
which  constitute  the  general  circulation  of  the 
atmosphere  is  due  to  a  French  scientist  Maurice 
de  Tastes,  whose  work  has  been  overlooked,  and 
whose  name  is  not  even  mentioned  in  certain 
treatises  on  meteorology.  This  conception  more- 
over did  not  pretend  to  give  details  but  only  a 
general  impression  of  the  atmospheric  circulation. 
The  complete  theory,  which  would  enable  us  to 
predict,  with  all  their  most  detailed  circumstances, 
every  meteorological  phenomena  without  excep- 
tion, has  yet  to  be  attained,  but  the  outline  given 
by  Maurice  de  Tastes  will  none  the  less  remain 
as  the  first  exact  general  representation  of  the 
movements  with  which  the  air  enveloping  us  is 
endowed. 

What  is  noteworthy  about  this  result  is  that 
it  enables  us  to  predict  occurrences  which  formerly 
were  difficult  enough  to  explain  after  the  event, 
viz. :  the  cyclones  of  the  tropical  regions  the  pro- 


Marine  Circulation  331 

duction  of  which  follows  naturally  from  the  con- 
ception of  aerial  current  circuits.  Let  us  consider 
in  fact  the  two  aerial  currents  which  carry  air 
along,  one  above  the  Gulf-Stream  in  the  North 
Atlantic,  the  other  above  Kuro-Siwo  in  the  North 
Pacific.  These  two  circuits  are  separated  from 
each  other  by  Texas  and  the  warm  lands  of  North 
America.  At  the  period  of  the  summer  solstice 
when  the  solar  temperature  attains  its  maximum, 
this  region  becomes  heated  more  rapidly  than  the 
neighbouring  sea.  Consequently,  a  movement 
of  ascension  will  be  produced  in  the  air  strata  lying 
above  it  and  thus  it  will  be  the  seat  of  a  depression. 
As  a  result  of  this  depression,  the  neighbouring 
air  masses  move  towards  the  region  and  the  atmo- 
spheric circuits,  Atlantic  and  Pacific,  separate  up 
to  this  point,  become  displaced,  and  so  come  into 
contact  with  each  other.  The  molecules  of  air 
that,  in  the  region  between  these  two  aerial  cur- 
rents, are  constrained  to  rotate  in  a  circle  in  the 
opposite  direction  to  the  hands  of  a  watch,  are 
forced  into  the  same  direction  of  rotation  both  by 
the  cyclonic  movement  due  to  the  local  depression 
and  by  the  rotation  couple  due  to  the  proximity 
of  the  portions  of  the  two  circuits  travelling  in 
opposite  directions.  A  cyclone  is  thus  produced 
and  the  phenomenon  occurs  each  time  the  con- 


332  The  Earth 

ditions  which  give  rise  to  it  recur.  Consequently, 
it  is  in  the  warm  season  and  in  the  regions  where 
the  two  neighbouring  circuits  can  meet  that  these 
rotary  storms  are  engendered.  We  can,  therefore, 
predict  that  cyclones  will  be  local  and  seasonal, 
and  observation  verifies  this  exactly.  An  admir- 
able confirmation  of  this  theory  has  been  afforded 
by  the  absence  of  cyclones  in  South  America  in 
spite  of  the  proximity  of  the  two  circuits  of 
the  South  Atlantic  and  the  South  Pacific,  for  the 
reason  that  between  the  two  the  Cordillera  of  the 
Andes  forms  an  effective  barrier.  Cyclones  are 
phenomena  which  do  not  attain  any  great  heights 
in  our  atmosphere;  at  2000  or  3000  metres  [1.25 
to  2.50  miles],  if  not  quite  absent,  they  are  at  any 
rate  greatly  weakened.  Therefore,  the  Cordillera, 
the  summits  of  which  rise  to  a  height  of  6000  and 
7000  metres  [3.75  to  4.25  miles]  and  the  mean 
altitude  of  which  in  the  Argentine  region  attains 
and  surpasses  3500  metres  [2.2  miles]  oppose  an 
insurmountable  obstacle  to  the  meeting  of  the 
Atlantic  and  the  Pacific  currents,  and,  consequently 
to  the  formation  of  cyclones  between  them. 

This  double  circulation,  atmospheric  and  marine, 
of  which  the  two  manifestations  are  so  directly 
connected,  is  completed  by  a  third  circulation 
which  is  the  consequence  of  the  two  first,  viz.: 


Marine  Circulation  333 

the  fluvial  circulation  which  returns  to  the  sea  the 
waters  that  the  solar  heat  has  removed  from  it  in 
the  form  of  vapour,  and  which  atmospheric  cur- 
rents assisted  by  oceanic  currents  have  trans- 
ported to  colder  continents,  where  it  has  been 
precipitated  as  rain  or  to  high  mountains  where 
it  has  been  condensed  as  snow. 

Everyone  knows  how  necessary  water  is. 
Where  there  is  none,  no  animal  or  vegetable  life 
is  found.  All  the  water  that  is  indispensable  to 
the  existence  of  organised  beings  and  indispensable 
also  to  that  industry  which  is  the  accompaniment 
of  life,  arises  from  the  condensation  of  atmospheric 
water-vapour  in  the  form  of  rain  and  snow.  If 
the  total  quantity  of  rain  falling  on  the  land  sur- 
face in  the  course  of  the  year  were  uniformly  dis- 
tributed, and  if  the  land  surface  were  everywhere 
quite  level  and  so  coincided  with  a  surface  con- 
centric with  the  geoid,  the  water  so  falling  would, 
at  the  end  of  a  year,  form  a  layer  85  centimetres 
[33.464  in.]  in  thickness,  which  implies,  considering 
the  area  of  the  continents,  an  equal  quantity  of 
rain  occupying  a  volume  of  122,500  cubic  kilo- 
metres [27,400  cub.  miles].  If  we  recall  that  the 
volume  of  the  water  of  all  the  oceans  is  about 
1300  million  cubic  kilometres  [312  million  cub. 
miles],  it  follows  that  the  total  annual  rainfall 


334  The  Earth 

represents  about  the  eleven-thousandth  part  of 
this. 

Only  a  portion  of  this  rainfall  is  restored  to  the 
seas  by  means  of  rivers.  These,  in  fact,  carry 
annually  28,000  cubic  kilometres  [6650  cub.  miles] 
of  water  to  the  sea,  scarcely  one  quarter  of  the 
total  water  precipitated  on  the  land  surface.  As 
regards  the  rest  of  the  rainfall,  part  is  evaporated, 
and  the  remainder  absorbed  by  the  ground  and 
by  living  beings.  Rivers  actually  restore  to  the 
oceans  only  48too1o.ooo  part  of  the  water  that  the 
Sun's  heat  had  raised  from  them  by  evaporation. 
This  fraction,  viz.:  the  28,000  cubic  kilometres 
[6650  cub.  miles],  is  all  that,  by  the  double  means 
of  the  atmospheric  circulation  and  the  pluvial 
circulation,  executes  a  constant  circuit  between 
the  seas  and  continents,  taking  back  to  the  oceanic 
reserves,  by  the  operation  of  gravity,  the  water 
they  lose  by  evaporation.  Rivers  thus  play  in 
some  measure  the  part  in  the  Earth's  economy 
that  blood-vessels  do  in  the  living  organism,  vessels 
that  lead  back  to  its  starting-point  the  blood  which 
has  been  taken  to  all  parts  of  the  body  and  which 
enables  it  to  live. 


CHAPTER  XI 

THE  ATTACK  ON  AND  DEFENCE  OF  THE  CONTINENTS 

IN  a  figurative  sense,  the  Earth  lives,  as  we  have 
*  seen.  Just  as  in  the  case  of  all  living  beings, 
there  are  possibilities  of  organic  degradation  and 
of  alteration  or  disturbance  of  function.  In  the 
course  of  the  preceding  chapter,  we  have  examined 
the  mechanism  of  the  terrestrial  circulation;  in 
the  present  one,  we  shall  see  by  what  agencies  the 
Earth's  surface  may  be  attacked  and  by  what 
means,  in  a  figurative  struggle  for  life,  these 
destructive  causes  are  counteracted. 

The  gaseous  mass  which  forms  the  atmosphere, 
and  the  liquid  mass  constituting  the  oceans,  are 
in  perpetual  movement  around  the  solid  crust 
that  is  the  external  envelope  of  the  Earth's  nucleus. 
This  crust,  far  from  being  uniform  and  level,  is 
furrowed  and  irregular,  traversed  by  valleys  and 
rugged  with  mountains.  We  have  to  consider 
what  happens  to  this  crust  in  the  presence  of  the 

335 


336  The  EartK 

moving  fluid  masses  which  unceasingly  assail  it. 
In  what  way,  and  up  to  what  limit,  can  the  solid 
material  constituting  the  crust  resist  the  attacks 
of  wind  and  water?  Are  there  innate  means  of 
defence  against  such  incessant  activity?  We  shall 
endeavour  to  elucidate  this  matter  in  the  following 
chapter. 

When  masses  of  air  in  the  equatorial  region  rise 
to  the  upper  strata  of  the  atmosphere  as  a  result 
of  the  diminution  of  density  caused  by  the  solar 
heat,  they  take  up  with  them  large  quantities  of 
water-vapour,  greater  in  proportion  to  the  eleva- 
tion of  temperature.  At  30°  C.  [86°  P.],  for 
example,  the  maximum  pressure  of  aqueous  vapour 
is  measured  by  31  millimetres  [1.22  in.]  of  mercury, 
and  consequently  represents,  in  a  saturated  atmo- 
sphere, one  twenty-fifth  part  of  the  total  atmo- 
spheric pressure.  Carried  along  with  the  air  by 
means  of  the  anti-trade  winds,  this  vapour  arrives 
over  the  cooler  countries  where  these  air  masses 
descend  down  to  the  Earth's  surface.  Further- 
more, the  masses  of  warm  humid  air  that  accom- 
pany the  oceanic  currents  and  pass  on  over  the 
continents  also  bring  considerable  quantities  of 
water-vapour  to  the  atmosphere  of  these.  As 
this  vapour  thus  arrives  at  a  region,  the  tempera- 
ture of  which  is  lower  than  that  which  corresponds 


THe  AttacK  on  tHe  Continents     337 

to  the  pressure  of  water-vapour  actually  in  the 
air,  some  of  it  rapidly  condenses,  first  in  the  form 
of  clouds,  then  rain,  and  finally  snow  if  the  con- 
densation occurs  at  a  sufficiently  low  temperature, 
which  is  always  the  case  on  the  summits  of  high 
mountains.  The  water  so  condensed  in  various 
forms  is  the  chief  agency  in  the  denudation  of 
the  land-surface. 

The  Sun  itself,  however,  begins  the  action.  Un- 
der the  heating  effect  of  its  rays  even  the  hardest 
rocks  become  heated  during  the  day.  As  they 
are  poor  conductors  of  heat,  the  heating  takes 
place  only  in  the  parts  directly  exposed  to  the 
Sun,  and  consequently  the  expansion  which  results 
from  the  rise  of  temperature  does  not  affect  the 
whole  mass  equally ;  molecular  effects  are  produced 
tending  to  destroy  the  cohesion  which  binds  the 
molecules  together.  Sooner  or  later,  the  strain, 
repeated  daily  and  arrested  at  night,  when  there 
is  a  corresponding  contraction,  becomes  greater 
than  the  molecular  cohesion  can  stand;  the  rock 
splits  and  cracks  and  instead  of  a  continuous 
surface  the  exposed  part  is  fissured  and  broken. 

Then  comes  the  rain.  Reversing  the  Latin 
proverb,  we  must  here  say:  post  Phcebium  nubila. 
The  condensed  water  falls  into  the  crevices  and 
cracks  of  the  rocks.  If  the  rocks  occupy  an  ele- 


338  THe  EartH 

vated  position,  for  example  the  top  of  a  mountain, 
the  water  does  not  remain  liquid,  but  freezes  in 
the  interstices  of  the  rocks.  In  freezing,  its 
volume  increases,  and,  thus,  with  an  irresistible 
force,  it  widens  the  fissures  and  still  further  separ- 
ates the  walls  of  rock  on  either  side.  This  is  the 
second  stage  of  the  destructive  action ;  after  having 
been  fissured  by  the  Sun's  action  the  rock  is  sub- 
sequently fractured  and  disintegrated  into  larger 
or  smaller  fragments. 

There  now  supervenes  another  destructive 
agency,  that  of  gravity.  At  the  period  of  forma- 
tion of  those  foldings  of  the  crust  which  were  the 
origin  of  mountains,  the  matter  composing  them 
was  elevated  above  the  mean  level  of  the  seas  by 
effects  resulting  from  the  more  or  less  recent  mani- 
festations of  internal  energy.  These  effects,  by 
thus  raising  the  rocky  masses  and  giving  them 
potential  energy,  for  the  time  being  triumphed 
over  the  force  of  gravity;  but  this  reasserts  itself 
as  soon  as  the  rocks  begin  to  disintegrate  and  lose 
the  cohesion  which  originally  bound  the  whole 
mass  together.  The  fragments  resulting  from  the 
action  of  freezing  water  fall  down  the  mountains 
and  form  a  talus  of  debris  at  its  base.  In  the 
descent  they  frequently  knock  against  each  other, 
and  these  shocks,  absorbing  a  part  of  their  energy, 


THe  Attach,  on  tKe  Continents     339 

end  by  distributing  them  in  the  form  of  a  natural 
slope  from  which  gravity  alone  will  not  suffice  to 
displace  them.  But,  now,  a  fourth  assailant 
comes  into  action,  viz. :  running  water  derived  from 
the  rainfall.  Always  subject  to  the  action  of 
gravity,  water  tends  to  fall  to  a  lower  level  than 
that  where  it  originally  is.  Now  solid  substances 
are  maintained  by  friction  in  more  or  less  steep 
slopes  according  to  the  degree  of  regularity  of  the 
fragments  composing  the  slope.  But  this  is  not 
the  case  with  fluids,  which  cannot  finally  remain 
at  rest  until  they  reach  a  place  where  the  free 
surface,  filling  suitable  hollows,  coincides  with  a 
level  surface  parallel  to  that  of  the  geoid. 

In  the  course  of  its  descent  towards  this  final 
level,  the  water  falls  with  more  or  less  swiftness 
and  vigour  according  to  the  steepness  of  the  slopes 
over  which  it  passes.  In  proportion  as  it  descends, 
it  takes  up  and  carries  along  particles  that  external 
physical  agencies  have  detached  from  the  land 
surface;  furthermore,  in  virtue  of  its  force  it 
hollows  out  a  channel  which  marks  its  course  and 
so  continuously  wears  away  the  solid  earth.  When 
a  drop  of  water,  which  represents  a  liquid  pro- 
jectile, unites  with  other  similar  drops  the  effect 
of  the  added  mass  is  multiplied  by  the  square  of 
the  velocity.  In  consequence  of  this,  the  stream 


340  TKe  Earth 

that  has  so  taken  birth,  and  which  gradually 
becomes  a  torrent,  eats  deeper  and  deeper  into 
the  initial  groove  in  which  it  flows,  and  it  carries 
down  the  material  so  removed.  There  are  many 
obstacles  in  its  path  however;  for  example,  it 
may  arrive  at  a  hollow  or  depression  in  the  ground 
that  has  no  outlet,  in  which  case  after  transforming 
this  into  a  lake  it  has  to  wear  down  the  edges. 
Also  it  may  have  to  fall,  in  the  form  of  cascades 
and  waterfalls,  over  any  rocks  that  are  too  hard 
to  be  worn  away  immediately  and  so  open  a 
passage  to  its  waters.  It  is  only  able  to  wear 
away  such  a  surface  very  gradually,  but  this  it 
does  by  the  constant  friction  of  its  waters,  aided 
by  the  continual  knocking  of  the  stones  and  pieces 
of  rocks  previously  broken  off  and  carried  along 
by  the  water. 

In  all  cases,  the  stream  becomes  smoother  and 
more  gentle  little  by  little;  it  retains  its  original 
impetuosity  only  among  mountains  where  there 
are  high  rocky  masses  which  intercept  the  waters. 
In  regions  of  average  altitude,  and  plains,  the 
violence  of  its  descent  gradually  slackens.  The 
constant  friction  exercised  by  the  bed  of  the  stream 
on  the  water,  which  always  grows  larger  in  volume 
by  the  contributions  of  affluent  streams,  tends  to 
retard  its  movement.  Also,  the  slope  diminishes 


The  Attack  on  the  Continents     341 

gradually  in  proportion  as  the  mass  of  water  in- 
creases, reducing  the  velocity  and  force  of  the 
stream.  The  wearing  away  of  solid  matter  from 
the  two  shores  also  influences  the  stream ;  the  rocks, 
sand,  and  stones  carried  along  knock  against  and 
exert  friction  on  the  irregularities  of  the  bottom 
of  the  stream  and  gradually  wear  them  smooth, 
all  the  time  decreasing  the  slope  of  the  river  in 
proportion  to  the  distance  from  the  mountains 
where  it  had  its  source — that  is,  in  proportion 
as  it  nears  the  sea. 

We  thus  see  that  stones,  gravel,  and  sand  or, 
generally,  alluvial  matter  which  water  has  de- 
posited on  lands  that  it  covered  at  a  certain  period 
represent,  together  with  mud,  the  products  of 
destruction  of  portions  of  the  crust  over  which  a 
river  formerly  flowed.  But  this  is  not  the  only 
way  in  which  running  water  wears  down  the 
relief  of  the  land-surfaces.  There  is  another 
means,  equally  effective,  which,  instead  of  acting 
in  a  continuous  manner  like  the  flowing  of  a 
stream  of  water,  acts  intermittently.  This  is 
infiltration.  On  very  inclined  slopes,  great  masses 
of  rock  and  earth  soaked  with  water  may  over- 
hang valleys  hollowed  by  the  stream.  If  the 
layer  which  supports  them  is,  for  example,  a  clay 
capable  of  sliding,  the  whole  mass  gives  way  and 


342  The  Earth 

falls  down  into  the  valley.  In  such  a  way  millions 
of  cubic  metres  [or  yards]  of  debris  of  all  sorts  may 
be  thrown  down  in  a  few  minutes ;  this  is  of  frequent 
occurrence  in  Switzerland. 

The  tributaries  of  a  river  also  carry  on  a  similar 
work  of  slow  and  continuous  destruction;  from 
the  smallest  steamlet  to  the  mightiest  of  rivers 
every  one  is  producing  disintegration  of  the  solid 
earth  over  which  it  flows.  The  result  of  this  in- 
cessant wearing  down,  prolonged  during  numerous 
centuries,  should  be  the  complete  destruction  of 
all  the  continents.  Under  the  action  of  gravity 
all  the  materials  resulting  from  their  demolition, 
carried  to  the  seas  by  running  water,  would  ac- 
cumulate in  the  form  of  sediments  at  rest  on  the 
oceanic  bottoms. 

We  thus  arrive  at  the  seeming  possibility  of  a 
complete  levelling  of  the  continental  relief  by  the 
action  of  running  water,  after  the  elapsing  of  a 
sufficiently  great  length  of  time.  And  there  is 
also  still  another  form  which  the  attack  upon  the 
solid  land  by  water  may  take,  viz.:  the  action  of 
glaciers. 

On  the  summits  of  high  mountains  rain  does 
not  fall:  on  account  of  the  low  temperature,  the 
result  of  the  altitude,  the  drops  of  water  condense 
and  then  solidify,  taking  a  crystalline  form  and 


TKe  AttacK  on  tHe  Continents    343 

falling  as  snow.  Snow  is  very  light,  because  of 
the  numerous  spaces  separating  the  branches  of 
its  beautiful  crystals,  and  so  it  is  easily  driven  by 
the  wind,  and  accumulates  in  masses  which  fall 
as  avalanches,  carrying  down  with  them  stones 
and  pieces  of  rock  detached  from  the  mountains. 
These  consequently  fall  into  the  valleys  and  ra- 
vines. The  snow  also  accumulates  in  these  hollows, 
and  under  the  pressure  exerted  by  the  superimposed 
layers  it  exhibits  the  phenomenon  of  congelation 
by  pressure.  It  becomes  transformed  into  com- 
pact ice,  so  that  a  kind  of  river  of  ice  occupies 
the  bottom  of  the  ravine,  a  river  which  slowly 
descends  under  the  thrust  of  the  upper  layers  of 
snow  and  ice.  The  velocity  is  small,  of  the  order 
of  one  metre  [or  yard]  daily.  Subsequent  ava- 
lanches fall  on  to  the  glacier  and  deposit  large 
rocks  thereon;  thus  the  frozen  river  or  glacier, 
to  give  it  its  proper  name,  carries  along  huge 
blocks  that  have  broken  off  the  mountains  from 
which  came  the  snow  that  gave  rise  to  it.  These 
blocks  become  aligned  along  the  edges  of  the 
glacier  in  the  form  of  very  characteristic  trains 
which  are  called  moraines.  When,  at  the  end  of 
its  course,  the  glacier  enters  regions  of  lower  alti- 
tude or  more  elevated  temperature,  so  that  the  ice 
permanently  melts,  these  rocks  are  deposited  just 


344  THe  Earth 

beyond  its  termination  where  they  constitute  a 
kind  of  embankment  or  barrier  which  has  received 
the  name  of  the  frontal  moraine.  This  frontal 
moraine  is  incessantly  traversed  by  the  rapid 
streams  of  water  produced  by  the  fusion  of  the 
ice  at  the  end  of  the  glacier.  The  constituent 
rocks  are  thus  rolled  one  against  another  when  the 
action  of  these  torrents  has,  after  a  time,  destroyed 
their  equilibrium  and  washed  out  the  stones  and 
gravel  which  supported  them  in  position. 

When  the  glacier  is  formed  of  great  thickness 
in  the  cold  regions  in  the  neighbourhood  of  the 
poles,  its  front  end  may  reach  down  to  the  sea. 
There,  enormous  blocks  break  from  it,  and  these 
are  carried  by  ocean  currents  towards  warmer 
regions.  In  this  way,  the  icebergs  are  formed, 
large  masses  of  ice  the  sunken  part  of  which  is 
eight  or  nine  times  greater  than  the  emergent 
part.  Some  icebergs  weigh  as  much  as  millions 
of  tons.  Icebergs  may  destroy  the  largest  ships, 
in  consequence  of  their  vast  size.  When  they 
melt  completely,  any  rocky  masses  that  they  have 
torn  from  the  land-surface  and  have  carried  with 
them,  fall  to  the  bottom  of  the  sea. 

During  winters  which  are  characterised  by 
abundant  snowfalls,  the  glacier  descends  farther, 
and  so  encroaches  on  the  valley,  while,  on  the  other 


The  AttacK  on  the  Continents     345 

hand,  after  unusually  dry  years  the  ice  recedes, 
laying  bare  the  lowest  part  of  its  bed.  It  is  then 
possible  to  see  the  destructive  action  that  its 
descent  has  produced;  it  seems  as  if  a  gigantic 
plane  had  been  passed  over  the  ground  that  was 
covered  by  the  ice.  All  the  rocks  and  stones  are 
rounded  as  if  they  had  been  turned,  and  on  their 
polished  surfaces  may  be  seen  grooves  and  scratches 
produced  by  the  action  of  small  harder  stones  that 
the  ice  has  caused  to  rub  against  them.  Thus, 
while  plains  and  low  valleys  are  subjected  to  river 
erosion,  the  mountains,  in  spite  of  their  height 
and  consequently  apparent  immunity  from  attack 
by  water,  are  assailed  and  disintegrated  by  the 
action  of  snow  and  ice.  Glaciers  carry  the  result- 
ing debris  to  the  end  of  their  course  and  finally 
the  fragments  are  subjected  to  the  action  of  the 
torrents  of  water  arising  from  the  melting  of  the 
ice  which  finally  carry  them  to  still  lower  regions. 
Thus,  low-lying  country  becomes  covered  with 
the  debris  of  the  highest  summits.  To  recapitu- 
late, watercourses  constitute  a  slow  but  sure  agency 
for  the  disintegration  of  the  continents,  the  debris 
of  which  they  ultimately  convey  to  the  sea. 

Not  only  does  water  wear  away  the  exposed 
surface  of  the  land  over  which  it  flows,  but  it  also 
has  another  and  more  insidious  destructive  action. 


346  The  Earth 

It  infiltrates  into  the  mass  of  the  Earth  and  tra- 
verses it  in  the  form  of  subterranean  streams  and 
channels.  In  the  course  of  this  it  dissolves  a 
quantity  of  material  from  the  strata  and  as  finally 
such  water  re-enters  the  ocean,  it  brings  to  the 
latter  substances  in  solution  derived  from  the  rocks 
through  which  it  has  flowed.  The  sea  is  thus 
the  vast  reservoir  in  which  accumulate  little  by 
little  the  materials  arising  from  the  demolition 
of  dry  land. 

It  might  be  thought  that  there  are  at  any  rate 
certain  regions  of  the  Earth's  surface  immune  from 
destruction  by  any  form  of  aqueous  agency,  viz: 
those  where  no  rain  ever  falls,  in  other  words  the 
deserts.  Sublatd  causd,  tollitur  effectus. 

Unfortunately  as  regards  the  relief  of  such 
regions,  although  rain  takes  no  part  in  their  dis- 
integration, the  wind  plays  an  equally  destructive 
r61e.  The  rocks  of  these  regions  are  subjected 
to  the  breaking-up  influence  of  the  alternate 
expansions  and  contractions  caused  by  the  solar 
heat  during  the  day  and  the  subsequent  cooler 
night.  The  results  of  disintegration,  fragments 
of  rocks  which  mutual  friction  has  ground  to  the 
condition  of  grains  of  sand,  are  carried  by  the 
wind  and  forced  into  hollows  where  they  accumu- 
late, or  frequently  they  may  be  piled  up  in  the 


THe  Attach,  on  tHe  Continents     347 

form  of  dunes.  There  hard  sand  grains  are  flung 
by  the  wind  against  the  emergent  rocks  and  so 
grind  them  down  little  by  little,  producing  the 
curious  phenomena  of  wind  erosion  so  well  de- 
scribed by  Professor  Velain.  Thus,  no  part  of 
the  land  surface  escapes  from  the  destructive 
effects  of  subaerial  denudation,  which  if  not  due 
to  water,  derived  from  the  condensation  of  atmo- 
spheric water  vapour,  forming  rain,  watercourses 
or  glaciers,  acts  through  the  instrumentality  of 
the  winds  which,  aided  by  the  grains  of  sand, 
gradually  wear  away  crags  and  mountains. 

We  have  just  considered  the  action  of  water 
flowing  over  the  Earth's  surface,  thus  forming  an 
immense  circulatory  system.  But  the  sea  is  not 
merely  a  passive  receptacle  for  the  waste  of  the 
land  brought  down  to  it  by  running  water.  The 
sea  also  directly  attacks  the  land;  raised  by 
the  tides,  agitated  by  waves  and  endowed  with 
a  progressive  motion  by  marine  currents,  it  exerts 
a  destructive  influence  along  the  shores  of  the 
land  surface.  The  force  of  the  liquid  masses  so 
projected  against  rocks  and  cliffs  is  enormous  and 
reaches  thirty  tons  per  square  metre  [or  yard]. 
It  is  estimated  that  the  whole  Earth  contains 
about  250,000  kilometres  [155,000  miles]  of  marine 
shores,  so  it  is  obvious  that  there  is  a  large  space 


348  The  EartK 

over  which  the  sea's  destructive  action  can  be 
exerted.  This  action  is  manifest  in  the  erosion  of 
the  hardest  granites,  as  seen  on  the  Brittany 
coasts,  by  the  rounding  of  enormous  blocks  that 
the  waves  pile  one  on  the  other,  by  the  piercing 
of  great  arches,  and  the  falling  of  whole  cliff 
surfaces,  as  may  be  observed  on  the  Normandy 
coasts.  Although  very  formidable,  yet  the  attack 
made  by  the  ocean  is  confined  to  the  region  along 
the  shores,  while  the  action  of  running  water  is 
felt  over  the  whole  area  of  an  extent  of  land  so 
that  the  destructive  effect  of  the  latter  is  eight 
or  nine  times  greater  than  that  of  the  sea. 

As  regards  the  statistical  expression  of  the  work 
of  destruction  of  the  continents,  geologists  have 
calculated  that  the  sum  total  of  solid  matter 
yearly  removed  from  the  land-surface  is  about  25 
cubic  kilometres  [6  cub.  miles].  We  have  seen, 
on  the  other  hand,  that  the  total  continental  mass 
elevated  above  the  level  of  the  geoid  is  about  100 
million  cubic  kilometres  [24  million  cubic  miles]. 
If  the  rate  of  denudation  be  assumed  constant, 
it  would,  thus,  require  four  million  years  to  erode 
the  whole  of  the  dry  land,  and  carry  to  the  sea 
the  debris  resulting  therefrom.  The  time  would 
actually  be  less  than  this,  for,  in  proportion  as  the 
solid  matter  accumulated  in  the  ocean,  so  would 


THe  Defence  of  tHe  Continents    349 

the  level  of  the  latter  rise,  consequently  diminish- 
ing the  volume  of  the  remaining  land.  We  may, 
therefore,  follow  Lapparent  in  estimating  the 
time  necessary  for  the  complete  destruction  of  the 
continental  relief  as  three  and  a  half  million  years. 

We  must  now  inquire  whether  the  continents 
have  no  power  of  defence  against  these  indefati- 
gable adversaries,  and  whether  they  are  not  able 
to  make  some  kind  of  a  struggle  for  life,  just  as 
men  and  all  living  beings  do,  instinctively  resisting 
the  germs  of  disease  and  disintegration. 

Such  is  the  case.  Since  the  debris  from  land 
destruction  accumulates  in  the  sea  it  is  built  up 
in  layers  on  the  bottom  and  so  by  the  slow  upward 
growth  of  the  sediment  new  strata  are  added  to 
those  which  originally  formed  the  solid  crust  of  the 
terrestrial  globe.  The  way  in  which  this  building- 
up  process  occurs  is  simple. 

When  the  sea  has  sapped  a  cliff  at  the  base  the 
fragments  resulting  from  its  fall  become  rounded 
by  the  action  of  the  waves  and  so  are  transformed 
into  shingle.  This  shingle  collects  in  a  line  at  the 
foot  of  the  cliff,  forming  an  enbankment  which 
extends  parallel  to  the  line  of  the  coast.  Beyond 
the  shingle,  and  washed  incessantly  by  the  waves 
which  alternately  advance  and  retreat,  are  the 
fine  sands  and  then  the  ooze  which  is  gradually 


350  The  Earth 

deposited  at  the  bottom  of  the  sea,  where  the 
violent  agitation  of  the  surface  is  felt  less  and  less 
in  proportion  to  the  depth. 

When  a  large  river  enters  into  a  tideless  sea 
the  water  bringing  down  the  sand  and  mud  de- 
posits these  materials  little  by  little  and  the  river 
mouth  becomes  gradually  blocked  by  the  con- 
tinual arrival  of  new  alluvial  matter.  A  direct 
passage  for  the  water  is  all  that  remains  in  the 
midst  of  the  sands  so  deposited  by  the  rivers. 
These  sands  extend  seawards  more  and  more, 
gradually  gaining  on  the  sea  and  forming  the 
characteristic  regions  that  may  be  seen  at  the 
mouths  of  the  Rhone,  the  Po,  the  Nile,  and  Missis- 
sippi, and  which  are  called  deltas.  The  vegetable 
remains  carried  down  by  the  river  accumulate 
in  these  deltas.  Subsequent  deposits  bury  them 
under  new  layers  of  mud  and  so  protect  them 
from  atmospheric  action,  transforming  them  into 
abundant  reserves  of  fuel  for  future  exploitation. 
If  a  river  enters  an  ocean  that  exhibits  marked 
tides,  instead  of  a  tideless  one  such  as  the  Medi- 
terranean or  Adriatic,  the  formation  of  a  delta  is 
difficult,  but  in  such  a  case  a  bar  is  gradually 
formed  by  means  of  the  periodical  conflict  between 
the  river  current  and  the  rising  tide.  A  bar  is  a 
submarine  embankment  of  mud  and  sand  which 


I  ' 
THe  Defence  of  tHe  Continents    351 

the  variations  of  the  tides  and  the  vicissitudes  to 
which  the  river  is  subjected  may  slowly  displace, 
and  which  always  constitutes  an  obstacle,  and 
sometimes  a  danger,  to  navigation  on  account  of 
its  submerged  character  and  the  breaking  of  waves 
upon  it. 

The  flood-tide  adds  daily  to  the  bar  what  is 
brought  down  by  the  river,  and  the  ebb-tide  carries 
the  rest  of  the  material  into  the  open  sea  where  it 
is  deposited  on  the  bottom  as  sand  and  ooze. 
Thus,  a  layer  of  deposits  is  formed  around  the 
continental  coasts,  material  being  laid  down  over 
an  average  breadth  of  200  kilometres  [124  miles]; 
near  the  coasts  it  is  formed  of  shingle  and  sand, 
but  farther  seawards  it  is  exclusively  composed 
of  very  fine  particles  of  pure  clay  which  constitutes 
marine  ooze.  In  this  way  a  sedimentary  deposit 
is  slowly  built  up,  gaining  more  than  a  millimetre 
[.039  in.]  annually  in  height. 

This  describes  what  occurs  along  marine  shores 
where  important  rivers  carry  down  materials 
arising  from  the  disintegration  of  solid  earth.  In 
the  midst  of  the  vast  marine  expanses  which  sepa- 
rate the  continents  from  each  other,  the  building 
up  of  solid  earth  occurs  by  other  means.  Tiny 
living  creatures  work  indefatigably  at  the  task 
of  reconstructing  new  land,  finding  the  necessary 


352  THe  EartH 

materials  for  their  work  in  the  dissolved  mineral 
matters  that  the  water  of  the  sea  has  received  from 
the  land.  They  use  these  substances  in  the  con- 
struction of  their  skeleton  or  their  shell.  These 
microscopic  sea-workers  are  the  polyzoa  which 
live  in  colonies  in  the  tropical  seas,  when  the  tem- 
perature of  these  does  not  fall  below  20°  C.  [68° 
P.],  in  the  superficial  regions  of  the  sea  down  to  a 
depth  of  30  or  40  metres  [100  to  130  ft.].  These 
little  beings  constituting  a  veritable  vegetation, 
for  their  colonies  take  arborescent  forms,  construct 
with  the  lime  extracted  from  the  sea  the  bases  on 
which  they  build  up  their  edifice.  As  this  increases 
in  height,  the  lower  portions  cease  to  live,  and 
only  the  chalk  structure  remains,  the  solidity  of 
which  increases  by  reason  of  the  debris  accumu- 
lated within  the  interstices  of  the  base  as  a  result 
of  the  waves  breaking  off  the  emergent  branches 
of  the  coral.  Little  by  little,  the  whole  is  trans- 
formed into  a  coral  reef  emerging  just  above  the 
level  of  the  sea  at  its  lowest,  and  so  constituting 
a  great  danger  to  navigation.  In  front  of  the 
north-east  of  Australia  a  veritable  rampart,  a 
coral  barrier,  exists. 

Often,  especially  in  the  Pacific,  where  the  mani- 
festations of  internal  energy  are  intense  and  nu- 
merous, submarine  eruptions  have  taken  place 


TKe  Defence  of  tHe  Continents    353 

which  have  lifted  up  volcanic  cones  to  the  surface 
of  the  sea.  These  would  have  been  quickly 
washed  away  by  the  agency  of  the  waves,  but  the 
polyzoa  have  gathered  round  the  emergent  crater 
and  formed  coral  girdles  or  rings,  the  debris  from 
which,  washed  off  by  the  sea,  accumulated  towards 
the  centre  and  so  formed  annular  islands,  the 
atolls,  several  of  which  have  become  habitable 
and  are  actually  inhabited  by  human  beings, 
who  thus  benefit  by  the  work  of  these  microscopic 
animals. 

Throughout  the  whole  of  the  sea  live  animal- 
culae  which  are  often  phosphorescent,  and  which 
also  extract  and  solidify  the  chalky  matter  that 
the  rivers  have  dissolved  from  the  land  and  carried 
to  the  oceans.  After  their  death,  the  chalky 
envelope  of  these  minute  creatures  falls  to  the 
bottom  of  the  sea  in  the  form  of  foraminiferal 
ooze  which  entirely  covers  the  bottom.  Other 
small  animals  act  similarly  with  regard  to  silica, 
and  their  debris  also  adds  to  the  accumulating 
sediment  on  the  oceanic  bottoms. 

However,  in  spite  of  all,  the  continents  must  end 
by  disappearing,  for  the  marine  shores  become 
bordered  little  by  little  with  debris;  the  level  of 
the  sea  rises  because  of  the  intrusion  of  solid  matter 
and  the  surviving  parts  of  the  continents  always 

23 


354  The  Earth 

continue  to  suffer  disintegration  by  atmospheric 
agencies  while  the  sea  rises  higher  and  higher. 
Finally,  therefore,  the  continents  must  completely 
disappear,  all  being  covered  by  the  water  of  the 
sea.  Yet  there  is  one  way  in  which  this  process 
is  retarded,  viz:  the  action  of  the  internal  energy 
manifested  as  volcanic  eruptions  and  seismic 
phenomena.  We  have  seen  in  the  course  of  the 
preceding  chapter  that  active  craters  eject  an 
enormous  amount  of  solid  matter  on  to  the  Earth's 
crust.  That  in  the  Sandwich  Islands  alone  has 
restored  to  the  crust  a  quantity  of  material  equal 
to  that  which  12,000  years  of  erosion  remove  from 
the  land  surface.  Taking  account  therefore  of  the 
total  number  of  existing  volcanoes  and  also  those 
that  have  existed  in  the  past  we  can  see  they 
constitute  a  considerable,  if  not  a  complete, 
compensation  for  continental  diminution  due  to 
erosion. 

Nevertheless,  it  must  be  remembered  that  the 
material  that  is  thus  ejected  above  the  superficial 
crust  disappears  from  the  central  magma  whence 
it  comes.  Consequently  the  crust,  ceasing  to  be 
sustained  at  every  point  must  sink  down  and  a 
sinking  even  to  the  extent  of  only  a  few  milli- 
metres [few  hundredths  of  an  inch]  all  over  the 
globe  would  be  a  greater  loss  to  the  dry  land  than 


THe  Defence  of  tHe  Continents    355 

the  entire  gain  brought  to  it  by  eruptions.  We 
have  seen  that  such  sinkings  occur,  they  produce 
seismic  phenomena  and  earth  convulsions.  These 
sinkings  may  be  partially  counterbalanced  by 
risings,  either  sudden  or  gradual  ones,  and  these 
always,  so  to  speak,  rejuvenate  the  exterior  sur- 
face of  the  crust  where  they  occur,  forming  new 
relief,  the  opposite  process  to  the  continual  wear- 
ing down  of  the  irregularities  of  the  crust  by  de- 
nudation. 


CHAPTER  XII 

THE  OLD  AGE  AND  DEATH  OF  THE  EARTH 

have  studied  the  "life"  of  the  Earth:  we 
have  pictured  its  birth  and  development 
and  have  learned  something  of  the  continual  circu- 
lation taking  place  upon  its  surface,  the  electric 
currents  which  traverse  its  mass,  and  the  tremors 
and  shocks  to  which  it  is  subjected.  Finally,  we 
have  considered  the  action  of  destructive  agents 
upon  the  land  surface  and  the  compensatory  influ- 
ences at  work,  analogous  to  those  which  defend 
a  living  being  against  the  attacks  of  germs  of 
disease.  But  a  healthy  being,  even  if  it  success- 
fully resist  the  efforts  of  morbid  action,  must 
finally  arrive  at  a  state  of  old  age.  With  age  the 
natural  forces  diminish,  the  circulation  slackens 
and  death  ensues,  an  accompaniment  of  the  cold 
that  succeeds  the  warmth  of  life. 

In  these  final  pages  we  shall  endeavour  to  find 
out  if  the  Earth  forms  any  exception  to  this  law, 
or  whether,  on  the  contrary,  it  also  grows  old  and 
in  due  course  dies. 

356 


Old  Age  and  Death  357 

In  the  first  place,  what  is  the  degree  of  per- 
manence of  the  actual  present  state  of  the  ter- 
restrial globe? 

The  conflict  between  the  land-surface  and  the 
exterior  agencies,  the  latter  tending  to  wear  it 
away  and  the  former  striving  to  defend  itself,  will 
endure  for  a  long  time  yet.  The  waters  will  have 
for  many  ages  to  continue  their  attack  against 
the  solid  material  brought  to  the  surface  of  the 
Earth  by  the  play  of  interior  forces,  and  the 
changes  brought  about  in  the  relative  position  of 
pre-existing  masses  by  seismic  disturbances. 

The  atmosphere  during  this  time,  or  at  least 
for  some  time,  will  grow  richer  in  carbonic  acid. 
On  the  one  hand,  volcanoes,  the  activity  of  which 
seems  to  be  actually  increasing  and  whose  mani- 
festations will  augment  in  number  in  proportion 
to  the  foldings  of  the  crust,  producing  new  fissures, 
set  free  an  abundance  of  this  gas.  On  the  other 
hand,  the  immense  progress  of  industry,  by  utilis- 
ing to  exhaustion  the  mineral  fuel  contained  in  the 
depths  of  the  Earth's  crust,  tends  to  increase  the 
proportion  of  carbonic  acid  in  the  atmosphere. 

For  some  time,  perhaps  a  very  long  one,  this 
proportion  will  augment.  Accordingly  the  influ- 
ence that  this  gas  exerts  as  regards  the  conserva- 
tion of  heat  will  also  increase,  and  this  will  protect 


358  The  Earth 

the  Earth  against  a  too  rapid  cooling.  Some  idea 
of  this  will  be  obtained  by  considering  that  if  the 
carbonic  acid  actually  contained  in  the  atmosphere 
viz:  jfoVo  part  of  the  atmosphere,  were  removed, 
the  temperature  of  the  Earth's  surface  would 
diminish  by  20°  C.  [36°  FJ,  and  this  diminution 
would  greatly  accentuate  the  present  climatic 
inequalities  of  the  various  regions  of  the  Earth. 
If,  on  the  contrary,  the  proportion  of  carbonic 
acid  were  to  increase, — for  example,  if  its  volume 
became  double, — we  should  experience  a  gain  of 
4°  C.  [7.2°  P.],  in  temperature,  and  8°  C.  [14.4°  P.], 
if  it  became  quadruple.  Furthermore,  not  only 
would  the  mean  temperature  rise,  but  there  would 
also  be  an  accompanying  tendency  to  climatic 
equalisation. 

The  study  of  the  Earth's  past  has  shown  us 
that  variations  of  this  *kind  formerly  occurred, 
and  had  an  influence  upon  the  phenomena  of 
animal  and  vegetable  life,  the  importance  of  which 
is  shown  by  geology.  If  the  carbonic  acid  in- 
creases, which  is  shown  by  the  continued  absorp- 
tion by  the  water  of  the  oceans,  above  which  the 
proportion  of  this  gas  in  the  atmosphere  is  one- 
tenth  less  than  above  the  land-surface,  these 
conditions  of  climatic  amelioration  would  be 
realised,  and  the  following  period  would  be  a 


Old  Ag'e  and  Death  359 

temperate  epoch,  in  the  course  of  which  there 
would  be  no  occasion  to  dread  the  recurrence  of 
those  terrible  glacial  periods  which  characterised 
the  beginning  of  the  Quaternary  era.  The  soil 
would,  thus,  increase  in  fertility,  for  the  rise  in 
temperature  of  the  air  above  it  would  increase  the 
quantity  of  water- vapour  contained  in  the  atmo- 
sphere, and  equally,  therefore,  the  abundance  of 
aqueous  precipitation.  Consequently,  there  would 
ensue  a  richer  vegetation  and  better  crops  for  the 
use  of  mankind  living  in  these  favoured  times. 

This,  however  will  only  be  a  temporary  allevi- 
ation of  the  Earth's  passage  towards  old  age 
and  death.  At  the  end  of  a  considerable  number 
of  centuries,  estimated  by  Helmholtz  at  17,000,000 
years, x  the  Sun  will  be  reduced  in  size  to  a  quarter 
of  its  actual  volume  on  account  of  the  loss  of  heat 
due  to  its  continued  radiation,  and  a  long  time 
before  this  has  taken  place  the  temperature  of 
the  Earth  will  not  exceed  zero  C.  [32°  FJ.  Life 
will  thus  not  last  for  the  whole  of  this  period — 
the  great  German  physicist  judged  its  ultimate 
duration  to  be  about  6,000,000  years. 

What  will  then  happen  to  the  Earth  itself  after 

'This  calculation  was  made  before  the  discovery  of  radio- 
activity, which,  by  maintaining  the  temperature  of  the  Sun  and 
Earth,  will  largely  retard  the  cooling  processes  described  in  this 
chapter. — Trans. 


360  The  Earth 

life  has  ceased  to  be  on  its  surface?  Will  Man, 
by  utilising  the  forces  of  Nature  and  the  future 
discoveries  of  science  that  will  continue  to  be 
made,  have  been  able  to  make  use  of  extra-terres- 
trial forces  and  so  postpone  this  state  of  affairs, 
or  even  to  betake  himself  to  other  and  newer 
worlds? 

In  the  process  of  the  gradual  cooling  of  the  Sun 
and  the  consequent  fall  of  the  terrestrial  tempera- 
ture, the  successive  stages  of  the  Earth's  state 
after  the  disappearance  of  life  from  its  surface  will 
be  as  follows.  The  oceans  and  rivers,  will  first 
become  transformed  into  masses  of  ice,  and  the 
clouds  having  condensed  into  snow  and  precipi- 
tated on  the  ground,  will  no  longer  afford  the 
Earth  the  protection  they  formerly  did  against 
loss  of  heat  by  radiation  into  space.  From  this 
time,  therefore,  the  temperature  will  fall  with 
greater  rapidity. 

Carbonic  acid  will  disappear  in  its  turn;  when 
the  temperature  is  sufficiently  low  it  will  fall  to 
the  ground  in  the  solid  form  as  a  fine  snow-like 
substance,  which  is  nowadays  employed  in  the 
laboratory  to  produce  cold.  This  condensation 
will  remove  the  last  defence  of  the  Earth  against 
radiation,  and  so  the  rate  of  cooling  from  that 
time  on  will  still  further  accelerate.  When  the 


Old  Age  and  Death  361 

temperature  reaches  73°  C.  absolute  [200°  C. 
below  the  ordinary  thermometric  zero  (328°  F.)], 
new  oceans  will  come  into  being,  and  will  accumu- 
late in  the  hollows  of  the  ice  which  covers  the 
planet.  These  new  oceans  will  be  produced  by 
the  liquefaction  of  nitrogen  and  oxygen  and  the 
remaining  atmosphere  will  consist  only  of  hydrogen 
and  helium,  and  will  be  in  a  state  of  extreme  ten- 
uity. The  cold  crust  will  thus  cover  a  globe 
exteriorly  inert,  but  the  interior  will  continue  to 
remain  as  magma  in  an  incandescent  state  for 
thousands  of  centuries.  A  very  small  portion 
only  of  this  heat  will  come  to  the  surface,  con- 
ducted through  the  crust,  which  gets  thicker  and 
thicker,  and  the  temperature  will  only  be  main- 
tained a bove  absolute  zero  [  -  273°  C.  or  -  459.4°  F.] 
by  the  radiations  received  from  the  cooling  sun, 
which  after  attaining  a  dull  red  condition  will 
also  finally  become  dark. 

Then  the  Sun  will  enter  on  its  final  period,  a 
superficial  crust  being  formed  by  solidification, 
just  as  occurred  in  the  case  of  the  Earth  at  the 
beginning  of  its  history.  At  first  it  will  be  a  very 
thin  skin,  constantly  broken  and  fissured  by  the 
force  of  the  internal  energy,  the  interior  lavas 
escaping  through  it,  but,  little  by  little,  the  solar 
crust  will  become  continuous. 


362  The  EartH 

From  that  moment  its  cooling  will  take  place 
more  rapidly  proportionally  to  the  relative  sizes  of 
the  two  bodies  than  that  of  the  Earth  at  the 
analogous  period  of  its  history  for  there  will  be 
no  body  at  all  to  supply  it  with  external  heat. 
In  a  continual  darkness,  illuminated  only  by  the 
light  of  the  distant  stars,  the  water-vapour  of  the 
solar  atmosphere  will  be  precipitated  on  its  sur- 
face and  form  oceans  there.  Relatively  soon  after 
their  formation  they  will  become  frozen.  The 
gases  of  the  solar  atmosphere  will  condense  in 
their  turn  and  the  Sun,  also,  will  then  be  a  globe, 
whose  interior,  containing  an  immense  reserve  of 
energy,  will  be  for  billions  of  centuries  prevented 
from  cooling  by  its  non-conducting  solid  crust. 
The  Sun  will  continue  its  journey  through  celestial 
space,  leading  with  it  its  cortege  of  superficially 
cooled  planets,  like  a  great  shell  charged  with  a 
terrible  explosive,  formed  by  the  endothermic 
compounds  accumulated  at  its  centre  and  main- 
tained at  a  temperature  of  several  million  degrees. 

The  Earth  also  will  be  in  a  similar  state,  but  on 
a  much  more  modest  scale,  continuing  to  gravitate 
around  its  former  Sun  with  an  interior  reserve 
of  energy  that  only  awaits  a  suitable  occasion  for 
renewed  manifestation,  for  liberation  with  the 
accompaniment  of  a  colossal  production  of  heat. 


Old  Age  and  Death  363 

The  collision  between  two  dark  bodies  in  inter- 
stellar space  appears  to  be  the  means  whereby 
the  rebirth  or  rejuvenation  of  a  world  takes  place. 
The  nearest  stars  are,  however,  at  so  great  a  dis- 
tance from  us  that  light,  although  travelling  with 
a  velocity  of  300,000  kilometres  [186,000  miles] 
per  second  takes  ten  years  to  traverse  it.  *  There- 
fore, as  our  Sun  is  journeying  towards  the  con- 
stellation of  Hercules  with  a  velocity  of  20 
kilometres  [12.5  miles]  per  second,  at  least  ten  bil- 
lion years  must  elapse  before  this  distance  can 
be  covered  and  a  collision  be  actually  possible. 

But  there  are  not  only  luminous  or  living  stars 
in  space.  We  have  supposed  our  Sun  extinct 
and  travelling  through  space.  It  may  encounter 
a  similar  body,  dark  and  therefore  invisible  to 
us,  situated  at  a  less  great  distance  than  the  stars 
we  see.  The  chances  that  such  an  encounter  may 
occur,  increase  with  the  decrease  of  distance 
between  the  two  bodies  on  account  of  the  attrac- 
tion, which  increases  in  proportion  to  the  square 
of  the  diminution  of  the  distance  which  separates 
them.  The  mathematical  theory  of  probabilities 
has  been  applied  to  this  subject,  and  it  is  found 
that  the  probable  time  elapsing  between  two 

'The  time  is  4.3  years  for  the  star  known  as  the  nearest- 
Alpha  Centauri;  8.1  years  for  the  next;  8.7  for  the  next;  10.1  for 
the  next.— Ed. 


364  THe  EartK 

successive  collisions  of  one  body  with  others  in  a 
million  million  years,  is  about  a  hundred  times 
as  long  as  the  duration  of  the  life  of  a  sun. 

It  has  been  calculated  that  the  meteorites  falling 
on  the  Sun  do  so  with  a  velocity  of  600  kilometres 
[362  miles]  per  second.  We  may  then  imagine 
our  two  celestial  bodies  coming  together,  each 
possessing  a  velocity  of  this  kind.  The  collision 
will  be  doubtless  oblique  for  the  chances  of  meet- 
ing normally  are  much  smaller.  The  shock  will 
thus  impress  on  the  moving  system  a  movement 
of  rotation,  the  peripheral  velocity  of  which  will 
be  enormous  and  will  attain  several  hundreds  of 
kilometres  [or  miles]  per  second. 

Even  if  the  two  bodies  so  colliding  were  entirely 
solid,  that  is  to  say,  if  they  were  cold  right  through 
to  the  centre,  the  tremendous  force  of  the  shock, 
transformed  into  heat,  would  suffice  to  volatilise 
completely  all  the  constituent  matter.  But  we 
know  that  they  may  be  and  probably  are  great 
reserves  of  energy,  full  of  endothermic  compounds, 
the  force  of  which  is  illustrated  by  the  velocity 
with  which  are  expelled  the  jets  that  actually  form 
the  solar  prominences.  This  energy  is  certainly 
relatively  thousands  of  times  greater  than  that  of 
our  most  terrible  modern  explosives.  As  to  the 
possibility  of  such  combinations,  the  continual 


Old  A&'e  and  DeatH  365 

disengagement  of  heat  from  radioactive  substances 
is  a  familiar  illustration.  Endothermic  combina- 
tions are  formed  throughout  the  evolution  of  suns 
during  their  period  of  brilliancy,  and,  in  all 
probability,  result  from  the  union  of  hydrogen 
and  helium  with  carbon  and  the  metals. 

When  a  collision  occurs  between  two  extinct 
suns  these  substances  are  decomposed  into  their 
ultimate  elements,  and  set  free  an  inconceivably 
vast  quantity  of  heat. 

Then  the  whole  mass  is  volatilised  to  give  birth 
to  what  we  call  a  new  star  or  nova,  such  for  example 
as  Nova  Persei.  Sometimes  perhaps  several  such 
bodies  might  result  from  the  impact,  being  sepa- 
rated from  the  original  agglomeration  of  incan- 
descent matter.  Two  gaseous  lateral  jets,  the 
result  of  the  obliquity  of  the  impact,  will  shoot 
forth  forming  a  spiral,  with  a  velocity  of  several 
hundred  kilometres  [or  miles]  per  second,  and  the 
gas  composing  them  will  constitute  the  spiral  arms 
of  a  new  nebula,  whose  nucleus  or  nuclei  will  be 
stars  in  process  of  birth.  Thus,  there  comes  into 
being  a  nebulous  system,  with  a  star  in  its  centre, 
and  all  the  phases  through  which  our  Sun  and 
planets  have  been  will  reoccur  in  the  new 
cycle. 

Thus  takes  place  the  resurrection  of    a  world. 


366  TKe  Earth 

And,  once  more,  on  the  great  dial  of  the  sky  where 
the  life  of  suns  is  the  measure  of  minutes,  the 
clock  of  eternity  will  have  accomplished  one  of 
its  turns. 


INDEX 


Abbadie.d',  178 

Amagat,  establishes  absolute 
zero,  16;  89 

Amundsen,  Roald,  surveys  of, 
in  Antarctic,  211;  243 

"Angle  of  depression,"  57 

Animals,  prehistoric,  41  et  seq. 

Apex  of  Sun's  way,  124,  125 

Archaean  rocks,  37  ff. 

Archimedes,  139 

Arrhenius,  Svante,  theory  of 
carbon  dioxide  in  atmo- 
sphere, 35,  36;  estimate  of 
Sun's  electrical  charge,  231; 
theory  of,  concerning  solar 
corona,  253;  9,  14, 15..38, 266 

Atmosphere,  composition  of, 
313;  carbon  dioxide  in,  35, 

357  ff« 
Atomic    weights,    their    effect 

upon  composition  of  Earth's 

crust,  25,  26 
Atwood's  machine,  131 
Auroras,    theories    concerning, 

254  et  seq. 


Bailie,  83 

Barometer,  the  aneroid,  146 

Bauer,  Louis  A.,  of  Carnegie 
Institution,  274 

Becquerel,  Henri,  discoverer  of 
radioactivity,  49;  260 

Bertrand,  Joseph,  quoted,  81 

Bessel,  Friedrich  Wilhelm,  de- 
termines flattening  of  Earth, 
156 

Birkeland,  Professor,  experi- 
ments of,  254 

Borda,  Jean  Charles,  135 


Bouguer,  Pierre,  measures 
Peru  arc,  220;  61,  79, 135 

Bourgeois,  Colonel,  remeasures 
Peru  arc,  220 

Brunhes,  Bernard,  confirms 
Folgheraiter,  240;  231,  250 

Calorie,  definition  of,  262 

Carbonic  acid,  theory  con- 
cerning, 35,  36;  effect  on 
Earth's  temperature,  357  ff. 

Carboniferous  period,  39 

Carnegie  Institution,  the,  274 

Cassini,  Jacques  Dominique, 
61 

Cavendish,  Henry,  determines 
mean  density  of  Earth,  81  ff. ; 
86 

Celsius,  Anders,  224 

Centrifugal  force,  law  of,  100; 
of  Earth's  rotation,  100  ff. 

Chandler,  Seth  Carlo,  116  ff. 

Charcot,  surveys  of,  in  Ant- 
arctic, 211 

Clairant,  Alexis  Claude,  156 

Columbus,  Christopher,  238 

Comets,  4;  tails  of,  10 

Compass  needle.  See  Mag- 
netic needle. 

Copernicus,  Nicholas,  92 

Coriolis,  theorem  of,  98 

Cornu,  83 

Corona,  solar.     See  Sun. 

Cretaceous  period,  41 

Crookes  tube,  15,  1 6 

Curie,  Mme.,  260 

Darwin,  Sir  George,  178,  292 

Darwin,  H.,  178 

Day,  the,  determination  of,  91 


367 


368 


Index 


De  Launay,  Charles  Eugene, 
25;  History  of  the  Earth,  36 

Democritus,  92 

Density  of  Earth,  86  ff. 

De  Prony,  137 

Deslandres,  125 

Deviation  of  vertical.  See 
Vertical,  deviation  of. 

Devonian  period,  39 

Disk,  "spurious,"  2  n. 

Doppler's  principle,  125 

Dyne,  definition  of,  85  ff. 

Earthquakes,  cause  of,  194 
Endothermic  compounds,  266, 

364,  365 

Eocene  period,  44 

Eotv&s,  Baron  Roland,  83; 
gravity  measuring  appara- 
tus of,  163 

Equinoxes,  precession  of,  108 

Etna,  Mt.,  197,  200 

Fizeau,  Hippolyte  Louis,  125 

Folgheraiter,  Giuseppe,  r  e  - 
searches  of,  239  ff. 

Foucault,  pendulum  experi- 
ment of,  93  ff. ;  invents  gyro- 
scope, 94  ff. 

"Foucault's  currents,"  232 

Gaillot,  176,  187 

Galileo,  92 

Gauss,  Karl  Friedrich,  274 

Geikie,  Sir  Archibald,  esti- 
mates age  of  Earth,  49 

Geodetic  Association,  60 

Geodetic  Institute  of  Potsdam, 
183 

Gerlache,  de,  surveys  of,  in 
Antarctic,  211 

Gravitation,  law  of,  universal, 
7;  constant  of,  85;  formula 
of  Newton's  Law,  85 ;  Jolly's 
experiment,  129;  dynamic 
method  of  determination, 
132  ff.;  absolute  value  of 
gravity,  149;  variation  of, 
formula  for,  155;  and  the 
destruction  of  the  continents, 
338;  92,  123,  124 


Green,    Lowthian,    theory   of, 

208;  219 
Greenwich,  112 
Grye,  Bouquet  de  la,  177 
Guillaume,  146 
Gulf  Stream,  322  ff. 
Gyroscope,  invention  of,  94  ff. 

Hahn,  Dr.,  120 

Hale,  George  Ellery,  125 

"Harmonic  analysis,"  292 

Hecker,  Dr.,  147;  gravity  mea- 
surements of,  1 66;  deter- 
mines deviation  of  vertical, 
183  ff. 

Heim,  162 

Helium,  discovery  of,  12;  trans- 
formed from  hydrogen,  13; 
percentage  of,  in  atmosphere, 
27;  18,  260  et  seq. 

Helmert,  determines  flattening 
of  the  Earth,  156;  65,  180 

Helmholtz,  Hermann  Ludwig, 
estimate  of,  concerning  the 
Sun,  359 

Herachtus,    92 

Herschel,  Sir  William,  12 

Hill,  determines  flattening  of 
the  Earth,  156 

History  of  the  Earth,  De 
Launay,  36 

Hutton,  Dr.  James,  80 

Hypsometer,  the,  146,  166 

Infra-red  rays,  8 
Intercontinental       depression, 

215 

International    Geodetic    Asso- 
ciation, 114,  219 
Isogonic  lines,  244  et  seq. 

Jean-Mayen,  200 
Jolly,  estimates  age  of  Earth, 
48;  experiment  of,  129,  156 
Jurassic  period,  41 

Kelvin,  William  Thomas,  Lord, 
determination  of  Earth's 
rigidity,  182;  invents  tide 
predicter,  295;  118,  174,  177, 
189,  292 


Inde: 


369 


Kepler,  Johann,  laws  of,  6,  7; 

8,  20,  102,  122,  235 
Koveslige"thy,  M.,  206 
Krakatoa,  198,  228 
Kuro-Siwo,  the,  325 

Labedeff    measures    radiation 

pressure,   9 
La  Condamine,  Charles  Marie, 

measures  Peru  arc,  220;  61, 

79,  .135 

Lacroix,  Professor,  195 

Lagrange,  Professor,  119 

Lallemand,  Charles,  65,  186, 
189,  208,  217 

Laplace,  Pierre  Simon,  nebu- 
lar hypothesis  of ,  14,  15;  1 6, 
20,  286 

Lapparent,  73,  192;  estimate 
of  time  for  destruction  of 
continents,  349 

Lassen  Peak,  208  n. 

Latitude,  fluctuation  of ,  112 

L&m,  178 

Life,  transmission,  of,  from 
other  worlds,  38 

Light,  velocity  of,  263 

Lippmann,  hypothesis  con- 
cerning Earth's  crust,  53, 
54,  167,  273,  274;  219 

Longitude,  standard  meridian 
of,  112 

Lowell  Observatory,  13 

Lubbock,  Sir  John  William, 
theory  of  tidal  transmission, 
300 

Magnetarium,  the,  devised  by 
Wilde,  description  of,  267 
et  seq. 

Magnetic  meridian,  235 

Magnetic  needle,  inclination  of, 
235 ;  declination  of,  236  ff. 

Magnetic  poles,  the,  242,  243 

Magnetic  storms,  247  ff. 

Mairan,  141 

Man,  appearance  of,  on  Earth, 

Marcuse,  114 
Maskelyne,  Nevil,  79 
Matt,  292 


Matteucci,  Carlo,  250 
Mauna  Loa,  the  volcano,  196 
Maupertuis,  Pierre  Louis,  61 
Maury,  Matthew  Fontaine,  58 
Maxwell,     Clerke,      discovers 

radiation  pressure,  9;  63 
Mean  solar  day.     See  Day. 
Messina,  earthquake  of,  204 
Metre,  the,  adoption  of,  61 
Michelson,  63 
Milky  Way,  composition  of,  3; 

II 

Miocene  period,  44 
Mohn,  Henrik,  147 
Monaco,  Prince  Albert  of,  maps 

bed  of  oceans,  71 ;  58,  274 
Moon,  shadow  of  Earth  on,  5; 
origin  of,  2 1,  22;  mass  of,  107; 
effect  on  Earth's  motion,  107 
ff.,  123;  effect  of,  on  shape  of 
Earth,  174;  and  deviation  of 
the  vertical,  176  ff.;  connec- 
tion of,  with  seismic  phen- 
omena, 205;  tidal  effect  of, 
281  et  seq 
Mount  Petee,  195 
Moureu,  Professor,  260 


Nansen,  Fridtjof,  326 

Nebulae,  distribution  of,  1 1 ;  as 
absorbers  of  heat  and  light, 
II  and  n.;  visibility  of,  11; 
forms  of,  ii,  12;  composition 
of,  12, 15;  origin  of,  12, 14;  ro- 
tation of,  I3n.;  temperature 
of,  14,  15,  16;  electrification 
of,  15 ;  evolution  of,  15  et  seq. ; 
luminosity  of,  16 

Nebulium,  12 

Neolithic  period,  47 

Neptune,  19,  26 

Newcomb,  Simon,  118 

New  stars.     See  Novae. 

Newton,  Sir  Isaac,  explains 
tides,  283;  6,  7,  129,  131 

Nodon,  discoverer  of  electric 
action  of  Sun's  rays,  231 

Novae,  or  new  stars,  4,  14,  18, 

365 
Nutation,  109 


370 


Index 


Oceanographical  Institute,  274 
Oceans,  depth  of,  28,  211,  279; 

volume  of,  277;  salt  of,  278; 

gold  in,  278;  temperature  of, 

279  ff. 

Oligocene  period,  44 
Orbit  of  Earth,  102  ff.,  122 


Paleolithic  period,  47 

Pasteur,  Louis,  quoted,  112 

Peary,  Admiral  Robert  E., 
soundings  made  by,  in  Arc- 
tic Sea,  211 

Pendulum,  determination  of 
mass  of  Earth  by  use  of,  76; 
in  measurement  of  gravity, 
134  et  seq.;  precision  of  pen- 
dulum measures  of  gravity, 
151;  the  horizontal,  183  ff. 

Permian  period,  39 

Picard,  Jean,  61 

Planets,  position  of,  5;  form  of, 
5 ;  laws  governing  motions  of, 
6,  7;  orbits  of,  6,  7;  rotations 
of,  19 

Pleistocene  period,  46 

Pliocene  period,  44 

Plumb  line,  determination  of 
Earth's  mass  by,  77 

Plutonic  rocks,  37 

Poincare*,  Henri,  determines 
flattening  of  the  Earth,  156 

Poles,  the,  rotation  of  Earth  at, 
97;  movements  of,  112  ff. 

Poynting,  83 

Precession  of  equinoxes.  See 
Equinoxes. 

Prehistoric  animals,  41  et  seq. 

Pressure  in  interior  of  Earth, 
88,  89 

Pressure  of  radiation.  See 
Radiation  pressure. 

Preston,  114 

Primary  era,  39  et  seq.,  55 

Ptolemy,  92 

Puiseux,  Victor,  176,  190 

Pythagoras,  92 


Quaternary  era,  45  et  seq. 


Radau  determines  flattening 
of  the  Earth,  156;  176 

Radiation,  pressure  of,  8  ff. ; 
discovered  by  Maxwell,  9; 
measured  by  Labedeff,  9; 
force  of,  9,  10 

Radioactivity,  50,  260  et  seq. 

Radium,  260  et  seq. 

Radius  vector,  definition  of,  7 

Ramsay,  Sir  William,  50,  260, 
263 

Ricco,  calculation  of,  concern- 
ing aurorae,  255 

Roche-Wiechert,  law  of,  87 

Ross,  Sir  John,  surveys  in 
Antarctic,  211 

Rotation  of  Earth,  90  ff . ;  dis- 
covery of,  92  and  n. ;  experi- 
mental proof  of,  93  ff. 

Rowland,  Henry  Augustus, 
discovers  Sun's  magnetic 
field,  231 

Rutherford,  Ernest,  50 


Santorin,  198 

Saros,  the,  298 

Scott,  Captain,  surveys  of,  in 
Antarctic,  211 

Secondary  era,  40  et  seq.,  55 

Seismic  waves,  speed  of,  221 

Shackelton,  Sir  Ernest,  sur- 
veys of,  in  Antarctic,  211; 
determines  position  of  South 
Magnetic  Pole,  243 

Sidereal  day.     See  Day. 

Silurian  period,  39 

Solar  day.     See  Day. 

Solar  system,  origin  of,  15; 
motion  of,  124 

Spectroscope,  use  of,  4;  in  de- 
termining stellar  velocities, 

125 

Spitaler,  Professor,  119 
"Spurious"  disk,  2  n. 
Star-clusters,  3 
Stars,  composition  of,  4;  ages  of, 

6;  colours  of,  6;  distribution 

of,    II ;   origins  of,    14,    15; 

collisions  of,  14,  18;  distance 

of,  363 


Index 


371 


Sterneck,  General  von,  me- 
thod of,  for  determining 
gravity,  141  ff.;  determines 
absolute  value  of  gravity, 
149 

Sun,  temperature  of,  3,  5; 
composition  of,  4,  25;  un- 
known elements  in,  4;  theory 
concerning  solar  corona,  10, 
253;  a  theory  concerning 
sun-spots,  53;  mass  of,  107, 
1 74 ;  effect  on  Earth's  motion, 
107  ff. ;  variation  of  magnetic 
influence  of,  120;  motion  of, 
124;  and  deviation,  of  the 
vertical,  174  ff.;  effect  of,  on 
shape  of  Earth,  174;  con- 
nection of,  with  seismic 
phenomena,  205 ;  electric 
action  of  rays  of,  231;  mag- 
netic field  of,  231  et  seq.\ 
radiation  of  electrified  par- 
ticles, 232  ff.;  effect  on 
auroras,  254  ff.;  effect  on 
terrestrial  magnetism,  274  ff . ; 
tidal  effect  of,  282  et  seq.; 
effect  on  continents,  337; 
estimate  of  duration  of,  359; 
cooling  of,  361,  362;  speed 
of,  motion  through  space, 
363;  collision  of,  with  star, 
363 

Tastes,  Maurice  de,  work  of, 

on  atmospheric  circulation, 

330 
Temperature  of  Earth,  increase 

proportional  to  depth,  88  and 

n. 

Teneriffe,  Peak  of,  200 
Tertiary  era,  44  ff.,  55 
Tetrahedron,  terrestrial,  208 

et  se. 


Thorium,  264 

Tidal  waves,  226  et  seq. 

Tide-predicter,  295  et  seq. 

Tides,  the,  280  et  seq. 

Time,  determination  of,  91 

Triassic  period,  41 

True  solar  day.     See  Day. 

Ultra-violet  rays,  8,  38 
Uranium,  263  ff. 
Uranus,  19,  26 

Vasilesco-Karpen,  confirms  fact 
of  magnetic  field  of  Sun,  232 
Velain,  Professor,  198,  347 
Vertical,  deviation  of  the,  174 

et  seq.,  180,  183  ff. 
Vesuvius,  Mount,  197 
Volcanoes,  cause  of,  194 
Volterra,  Professor,  118 

Water,  critical  temperature  of, 

28 
Wefforges,  General,  determines 

absolute  value    of    gravity, 

149 

Whewell,  theory  of  tidal  trans- 
mission, 300 

Wiechert-Roche,  law  of,  87 
Wilde,   M.  H.,  theory  of,  on 

terrestrial    magnetism,    267 

et  seq. 

Wolff,  Professor  C.,  178 
Wullersdorff-Urbair,       Count, 

gravity-measuring     method 

of,  145 

Xime"nes,  Father,  129 

Zero,  absolute,  16 

Zollner,  177 

Zones,  geographical,  315  ff. 


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